Table of contents

We specifically study one aspect of foreground primordial matter density perturbations: the relative gravitational time delay between a pair of light paths converging toward an observer and originating from two points on the last scattering surface separated by the physical scale of an acoustic oscillation. It is found that time delay biases the size of acoustic oscillations systematically toward smaller angles, or larger harmonic numbers l; that is, the mean geometry as revealed by light from the cosmic microwave background becomes that of an open universe if Ω = 1. Since the effect is second-order, its standard deviation δl/l ~ (δΦ)², where (δΦ)² ~ 10⁻⁹ is the normalization of the primordial matter spectrum P(k), the consequence is too numerically feeble to warrant a reinterpretation of WMAP data. If, however, this normalization were increased to δΦ ≳ 0.01, which is still well within the perturbation limit, the shift in the positions of the acoustic peaks would be substantial enough to implicate inflationary ΛCDM cosmology. Thus, Ω is not the only parameter (and, by deduction, inflation cannot be the only mechanism) of relevance to the understanding of observed large-scale geometry. The physics that explains why δΦ is so small also plays a crucial role, but since this is a separate issue, independent of inflation, might it be less artificial to look for an alternative solution to the flatness problem altogether?

In the nonlinear regime of cosmic clustering, the mass density field of the cosmic baryon fluid is highly non-Gaussian. It shows different dynamical behavior from collisionless dark matter. Nevertheless, the evolved field of the baryon fluid is scale-covariant in the range from the Jeans length to a few tens of h⁻¹ Mpc, within which the dynamical equations and initial perturbations are scale free. We show that in the scale-free range, the non-Gaussian features of the cosmic baryon fluid, governed by the Navier-Stokes equation in an expanding universe, can be well described by a log-Poisson hierarchical cascade. The log-Poisson scheme is a random multiplicative process (RMP), which causes non-Gaussianity and intermittency even when the original field is Gaussian. The log-Poisson RMP contains two dimensionless parameters: β for the intermittency and γ for the most singular structure. All the predictions given by the log-Poisson RMP model, including the hierarchical relation, the order dependence of the intermittent exponent, the moments, and the scale-scale correlation, are in good agreement with the results given by hydrodynamic simulations of the standard cold dark matter model. The intermittent parameter β decreases slightly at low redshift and indicates that the density field of the baryon fluid contains more singular structures at lower redshifts. The applicability of the model is addressed.

We study the importance of baryonic physics on predictions of the matter power spectrum as it is relevant for forthcoming weak-lensing surveys. We quantify the impact of baryonic physics using a set of cosmological numerical simulations. Each simulation has the same initial density field, but models a different set of physical processes. We find that baryonic processes significantly alter predictions for the matter power spectrum relative to models that include only gravitational interactions. Our results imply that future weak-lensing experiments such as LSST and SNAP will likely be sensitive to the uncertain physics governing the nonlinear evolution of the baryonic component of the universe if these experiments are primarily limited by statistical uncertainties. In particular, this effect could be important for forecasts of the constraining power of future surveys if information from scales l ≳ 1000 is included in the analysis. We find that deviations are caused primarily by the rearrangement of matter within individual dark matter halos relative to the gravity-only case, rather than a large-scale rearrangement of matter. Consequently, we propose a simple model, based on the phenomenological halo model of dark matter clustering, for baryonic effects that can be used to aid in the interpretation of forthcoming weak-lensing data.

At the epoch of reionization, when the high-redshift intergalactic medium (IGM) is being enriched with metals, the 63.2 μm fine-structure line of O I is pumped by the ~1300 Å soft UV background and introduces a spectral distortion in the cosmic microwave background (CMB). Here we use a toy model for the spatial distribution of neutral oxygen in which metal bubbles surround dark matter halos, and compute the fluctuations of this distortion and the angular power spectrum it imprints on the CMB. We discuss the dependence of the power spectrum on the velocity of the winds polluting the IGM with metals, the minimum mass of the halos producing these winds, and the cosmic epoch when the O I pumping occurs. We find that, although the clustering signal of the CMB distortion is weak [(δy)_rms ≲ 10⁻⁷; roughly corresponding to a temperature anisotropy of ~1 nK], it may be reachable in deep integrations with high-sensitivity infrared detectors. Even without a detection, these instruments should be able to set useful constraints on the heavy-element enrichment history of the IGM.

We present the results of polarimetric (R-band) and multicolor photometric (BH RI J H K) observations of the blazar AO 0235+16 during an outburst in 2006 December. The data reveal a short timescale of variability (several hours), which increases from optical to near-IR wavelengths; even shorter variations are detected in polarization. The flux density correlates with the degree of polarization, and at the maximum degree of polarization the electric vector tends to align with the parsec-scale jet direction. We find that a variable component with a steady power-law spectral energy distribution and very high optical polarization (30%-50%) is responsible for the variability. We interpret these properties of the blazar within a model of a transverse shock propagating down the jet. In this case a small change in the viewing angle of the jet, by ≲ 1°, and a decrease in the shocked plasma compression by a factor of ~1.5 are sufficient to account for the variability.

We present a new method to directly map the neutral-hydrogen distribution during the reionization epoch and constrain the emission properties of the highest redshift quasars (QSOs). As a tracer of H I, we propose to use the Lyα radiation produced by quasar ionization fronts (I-fronts) that expand in the partially ionized intergalactic medium (IGM) before reionization is complete. These Lyα photons are mainly generated by H I collisional excitations. Combining two radiative transfer models (one for the QSO ionizing radiation and one for the Lyα photons), we estimate the expected Lyα spectral shape and surface brightness (SB_Lyα) for a large number of configurations where we varied both the properties of the ionizing QSO and of the surrounding medium. We find that the expected signal is observable as a single (broad) emission line with a characteristic width of 100-200 km s⁻¹. The expected SB_Lyα produced at redshift z ≳ 6.5 within a fully neutral region (at mean density) by a typical QSO I-front lies in the range 10⁻²¹ to 10⁻²⁰ ergs s⁻¹ cm⁻² arcsec⁻² and decreases proportionally to (1 + z)² for a given QSO age. QSOs with harder spectra may produce a significantly brighter emission at early phases. The signal may cover up to a few hundred square arcminutes on the sky and should be already detectable with current facilities by means of moderate- to high-resolution spectroscopy. The detection of this Lyα emission can shed new light on the reionization history, the age and the emission properties of the highest redshift QSOs.

We analyze the gas kinematics of damped Lyα systems (DLAs) hosting high-z gamma-ray bursts (GRBs) and those toward quasars (QSO-DLAs), focusing on three statistics: (1) Δv₉₀, the velocity interval encompassing 90% of the integrated optical depth, and (2) W₁₅₂₆ and (3) W₁₅₄₈, the rest equivalent widths of the Si II 1526 and C IV 1548 transitions, respectively. The Δv₉₀ distributions of the GRB-DLAs and QSO-DLAs are similar; each has median Δv₉₀ ≈ 80 km s⁻¹ and a significant tail, extending to several hundred km s⁻¹. This suggests comparable galaxy masses for the parent populations of GRB-DLAs and QSO-DLAs, and we infer that the average dark matter halo mass of GRB galaxies is ≲10¹²M_☉. The unique configuration of GRB-DLA sight lines and the presence (and absence) of fine-structure absorption together give special insight into the nature of high-z protogalactic velocity fields. The data support a scenario in which the Δv₉₀ statistic reflects dynamics in the interstellar medium (ISM) and W₁₅₂₆ traces motions outside the ISM (e.g., halo gas and galactic-scale winds). The W₁₅₂₆ statistic and gas metallicity [M/H] are tightly correlated, especially for the QSO-DLAs: [M/H] = a + blog(W₁₅₂₆/1Å) with a = −0.92 ± 0.05 and b = 1.41 ± 0.10. We argue that the W₁₅₂₆ statistic primarily tracks dynamical motions in the halos of high-z galaxies and interpret this correlation as a mass-metallicity relation with very similar slope to the trend observed in local, low-metallicity galaxies. Finally, the GRB-DLAs exhibit systematically larger W₁₅₂₆ values (>0.5 Å) than the QSO-DLAs (⟨W₁₅₂₈⟩ ≈ 0.5 Å), which may suggest that galactic-scale outflows contribute to the largest observed velocity fields.

We performed relativistic magnetohydrodynamic simulations of the hydrodynamic boosting mechanism for relativistic jets explored by Aloy and Rezzolla using the RAISHIN code. Simulation results show that the presence of a magnetic field changes the properties of the shock interface between the tenuous, overpressured jet (V^z_j) flowing tangentially to a dense external medium. Magnetic fields can lead to more efficient acceleration of the jet, in comparison to the pure hydrodynamic case. A "poloidal" magnetic field (B^z), tangent to the interface and parallel to the jet flow, produces both a stronger outward moving shock and a stronger inward moving rarefaction wave. This leads to a large velocity component normal to the interface in addition to acceleration tangent to the interface, and the jet is thus accelerated to larger Lorentz factors than those obtained in the pure hydrodynamic case. Likewise, a strong "toroidal" magnetic field (B^y), tangent to the interface but perpendicular to the jet flow, also leads to stronger acceleration tangent to the shock interface relative to the pure hydrodynamic case. Overall, the acceleration efficiency in the poloidal case is less than that of the toroidal case, but both geometries still result in higher Lorentz factors than the pure hydrodynamic case. Thus, the presence and relative orientation of a magnetic field in relativistic jets can significantly modify the hydrodynamic boost mechanism studied by Aloy and Rezzolla.

We present a two-dimensional grid-based hydrodynamic simulation of a thin, viscous, locally isothermal corotating disk orbiting an equal-mass Newtonian binary point mass on a fixed circular orbit. We study the structure of the disk after multiple viscous times. The binary maintains a central hole in the viscously relaxed disk with radius equal to about twice the binary semimajor axis. Disk surface density within the hole is reduced by orders of magnitude relative to the density in the disk bulk. The inner truncation of the disk resembles the clearing of a gap in a protoplanetary disk. An initially circular disk becomes elliptical and then eccentric. Disturbances in the disk contain a component that is stationary in the rotating frame in which the binary is at rest; this component is a two-armed spiral density wave. We measure the distribution of the binary torque in the disk and find that the strongest positive torque is exerted inside the central low-density hole. We make connection with the linear theory of disk forcing at outer Lindblad resonances (OLRs) and find that the measured torque density distribution is consistent with forcing at the 3:2 (m = 2) OLR, well within the central hole. We also measure the time dependence of the rate at which gas accretes across the hole and find quasi-periodic structure. We discuss implications for variability and detection of active galactic nuclei containing a binary massive black hole.

We make use of deep HST, VLT, Spitzer, and Chandra data on the Chandra Deep Field-South to constrain the number of Compton-thick AGNs in this field. We show that sources with high 24 μm-to-optical flux ratios and red colors form a distinct source population, and that their infrared luminosity is dominated by AGN emission. Analysis of the X-ray properties of these extreme sources shows that most of them (80% ± 15%) are indeed likely to be highly obscured, Compton-thick AGNs. The number of infrared-selected, Compton-thick AGNs with 5.8 μm luminosity higher than 10^44.2 ergs s⁻¹ turns out to be similar to that of X-ray-selected, unobscured, and moderately obscured AGNs with 2-10 keV luminosity higher than 10⁴³ ergs s⁻¹ in the redshift bin 1.2-2.6. This "factor of 2" source population is exactly what is needed to solve the discrepancies between model predictions and X-ray AGN selection.

Outflows from active galactic nuclei (AGNs) seem to be common and are thought to be important from a variety of perspectives: as an agent of chemical enhancement of the interstellar and intergalactic media, as an agent of angular momentum removal from the accreting central engine, and as an agent limiting star formation in starbursting systems by blowing out gas and dust from the host galaxy. To understand these processes, we must determine what fraction of AGNs feature outflows and understand what forms they take. We examine recent surveys of quasar absorption lines, reviewing the best means to determine if systems are intrinsic and result from outflowing material, and the limitations of approaches taken to date. The surveys reveal that, while the fraction of specific forms of outflows depends on AGN properties, the overall fraction displaying outflows is fairly constant, approximately 60%, over many orders of magnitude in luminosity. We emphasize some issues concerning classification of outflows driven by data type rather than necessarily the physical nature of outflows and illustrate how understanding outflows probably requires a more comprehensive approach than has usually been taken in the past.

The intrinsic fraction of broad absorption line quasars (BALQSOs) is important in constraining geometric and evolutionary models of quasars. We present the fraction of BALQSOs in 2MASS-detected quasars within the SDSS DR3 sample in the redshift range of 1.7 ⩽ z ⩽ 4.38. The fraction of BALQSOs is 40.4^{+ 3.4}_−3.3% in the 2MASS 99% database K_s-band completeness sample, and 38.5^{+ 1.7}_−1.7% in the larger 2MASS sample extending below the completeness limit. These fractions are significantly higher than the 26% reported in the optical bands for the same parent sample. We also present the fraction of BALQSOs as functions of apparent magnitudes, absolute magnitudes, and redshift in the 2MASS and SDSS bands. The 2MASS fractions are consistently higher than the SDSS fractions in every comparison, and the BALQSO fractions steadily increase with wavelength from the SDSS u to the 2MASS K_s bands. Furthermore, the i − K_s color distributions of BALQSOs and non-BALQSOs indicate that BALQSOs are redder than non-BALQSOs, with a K-S test probability of 2 × 10⁻¹². These results are consistent with the spectral difference between BALQSOs and non-BALQSOs including both the absorption troughs and dust extinction in BALQSOs, which leads to significant selection biases against BALQSOs in the optical bands. Using a simple simulation incorporating the luminosity function of quasars and the amount of obscuration for BALQSOs, we simultaneously fit the BALQSO fractions in the SDSS and 2MASS bands. We obtain a true BALQSO fraction of (43 ± 2)% for luminous quasars (M_Ks ≲ −30.1 mag).

We analyze a sample of 58 multiwavelength, Very Long Baseline Array observations of active galactic nuclei (AGNs) to determine their scattering properties. Approximately 75% of the sample consists of AGNs that exhibit centimeter-wavelength intraday variability (interstellar scintillation), while the other 25% do not show intraday variability. We find that interstellar scattering is measurable for most of these AGNs, and the typical broadening diameter is 2 mas at 1 GHz. We find that the scintillating AGNs are typically at lower Galactic latitudes than the nonscintillating AGNs, consistent with the scenario that intraday variability is a propagation effect from the Galactic interstellar medium. The magnitude of the inferred interstellar broadening measured toward the scintillating AGNs, when scaled to higher frequencies, is comparable to the diameters inferred from analyses of the light curves for the more well-known intraday variable sources. However, we find no difference in the amount of scattering measured toward the scintillating versus nonscintillating AGNs. A consistent picture is one in which the scintillation results from localized regions ("clumps") distributed throughout the Galactic disk, but that individually make little contribution to the angular broadening. Of the 58 AGNs observed, 37 (64%) have measured redshifts. At best, a marginal trend is found for scintillating (nonscintillating) AGNs to have smaller (larger) angular diameters at higher redshifts. We also use our observations to try to constrain the possibility of intergalactic scattering. While broadly consistent with the scenario of a highly turbulent intergalactic medium, our observations do not place significant constraints on its properties.

We present a precise estimate of the bulk virial scaling relation of halos formed via hierarchical clustering in an ensemble of simulated cold dark matter cosmologies. The result is insensitive to cosmological parameters; the presence of a trace, dissipationless gas component; and numerical resolution down to a limit of ~1000 particles. The dark matter velocity dispersion scales with total mass as log [σ_DM(M,z)] = = log(1082.9 ± 4.0 km s⁻¹) + (0.3361 ± 0.0026)log[h(z)M₂₀₀/10¹⁵M_☉], with h(z) being the dimensionless Hubble parameter. At fixed mass, the velocity dispersion likelihood is nearly lognormal, with scatter σ_{ln σ} = 0.0426 ± 0.015, except for a tail with higher dispersions containing 10% of the population that are merger transients. We combine this relation with the halo mass function in ΛCDM models and show that a low normalization condition, S₈ = σ₈(Ω_m/0.3)^0.35 = 0.69, favored by recent WMAP and SDSS analysis requires that galaxy and gas-specific energies in rich clusters be 50% larger than that of the underlying dark matter. Such large energetic biases are in conflict with the current generation of direct simulations of cluster formation. A higher normalization, S₈ = 0.80, alleviates this tension and implies that the hot gas fraction within r₅₀₀ is , a value consistent with recent Sunyaev-Zel'dovich observations.

We present results from the Chandra X-ray observation of Abell 13, a galaxy cluster that contains an unusual noncentral radio source, also known as a radio relic. This is the first pointed X-ray observation of Abell 13, providing a more sensitive study of the properties of the X-ray gas. The X-ray emission from Abell 13 is extended to the northwest of the X-ray peak and shows substructure indicative of a recent merger event. The cluster X-ray emission is centered on the bright galaxy H of Slee et al. We find no evidence for a cooling flow in the cluster. A knot of excess X-ray emission is coincident with the other bright elliptical galaxy F. This knot of emission has properties similar to the enhanced emission associated with the large galaxies in the Coma Cluster. With these Chandra data we are able to compare the properties of the hot X-ray gas with those of the radio relic from VLA data, to study the interaction of the X-ray gas with the radio-emitting electrons. Our results suggest that the radio relic is associated with cooler gas in the cluster. We suggest two explanations for the coincidence of the cooler gas and radio source. First, the gas may have been uplifted by the radio relic from the cluster core. Alternatively, the relic and cool gas may have been displaced from the central galaxy during the cluster merger event.

We present the results of NICMOS imaging of two massive galaxies photometrically selected to have old stellar populations at z ∼ 2.5. Both galaxies are dominated by apparent disks of old stars, although one of them also has a small bulge comprising about one-third of the light at rest-frame 4800 Å. The presence of massive disks of old stars at high redshift means that at least some massive galaxies in the early universe have formed directly from the dissipative collapse of a large mass of gas. The stars formed in disks like these may have made significant contributions to the stellar populations of massive spheroids at the present epoch.

We present measurements of the color and luminosity dependence of galaxy clustering at z ∼ 1 in the DEEP2 Galaxy Redshift Survey. Using volume-limited subsamples in bins of both color and luminosity, we find the following: (1) The clustering dependence is much stronger with color than with luminosity and is as strong with color at z ∼ 1 as is found locally. We find no dependence of the clustering amplitude on color for galaxies on the red sequence, but a significant dependence on color for galaxies within the blue cloud. (2) For galaxies in the range L/L^* ∼ 0.7–2, a stronger large-scale luminosity dependence is seen for all galaxies than is seen for red and blue galaxies separately. The small-scale clustering amplitude depends significantly on luminosity for blue galaxies, with brighter samples having a stronger rise on scales r_p < 0.5 h⁻¹ Mpc. (3) Redder galaxies exhibit stronger small-scale redshift-space distortions ("fingers of god"), and both red and blue populations show large-scale distortions in ξ (r_p,π) due to coherent infall. (4) While the clustering length, r₀, increases smoothly with galaxy color (in narrow bins), its power-law exponent, γ, exhibits a sharp jump from the blue cloud to the red sequence. The intermediate-color "green" galaxy population likely includes transitional galaxies moving from the blue cloud to the red sequence; on large scales green galaxies are as clustered as red galaxies but show infall kinematics and a small-scale correlation slope akin to the blue galaxy population. (5) We compare our results to a semianalytic galaxy formation model applied to the Millennium Run simulation. Differences between the data and the model suggest that in the model star formation is shut down too efficiently in satellite galaxies.

We present the quantitative rest-frame B morphological evolution and galaxy merger fraction at 0.2 < z < 1.2 as observed by the All-Wavelength Extended Groth Strip International Survey (AEGIS). We use the Gini coefficient and M₂₀ to identify major mergers and classify galaxy morphology for a volume-limited sample of 3009 galaxies brighter than 0.4L^*_B, assuming pure luminosity evolution. We find that the merger fraction remains roughly constant at 10% ± 2% for 0.2 < z < 1.2. The fraction of E/S0/Sa galaxies increases from 21% ± 3% at z ∼ 1.1 to 44% ± 9% at z ∼ 0.3, while the fraction of Sb-Ir galaxies decreases from 64% ± 6% at z ∼ 1.1 to 47% ± 9% at z ∼ 0.3. The majority of z < 1.2 Spitzer MIPS 24 μm sources with L(IR) > 10¹¹L_☉ are disk galaxies, and only ~15% are classified as major merger candidates. Edge-on and dusty disk galaxies (Sb-Ir) are almost a third of the red sequence at z ∼ 1.1, while E/S0/Sa make up over 90% of the red sequence at z ∼ 0.3. Approximately 2% of our full sample are red mergers. We conclude (1) the merger rate does not evolve strongly between 0.2 < z < 1.2; (2) the decrease in the volume-averaged star formation rate density since z ∼ 1 is a result of declining star formation in disk galaxies rather than a disappearing population of major mergers; (3) the build-up of the red sequence at z < 1 can be explained by a doubling in the number of spheroidal galaxies since z ∼ 1.2.

We examine the faint-end slope of the rest-frame V-band luminosity function (LF), with respect to galaxy spectral type, of field galaxies with redshift z < 0.5, using a sample of 80,820 galaxies with photometric redshifts in the 2 deg² Cosmic Evolution Survey (COSMOS) field. For all galaxy spectral types combined, the LF slope ranges from –1.24 to –1.12, from the lowest redshift bin to the highest. In the lowest redshift bin (0.02 < z < 0.1), where the magnitude limit is M_V≲ − 13, the slope ranges from α ∼ − 1.1 for galaxies with early-type spectral energy distributions (SEDs) to α ∼ − 1.9 for galaxies with low-extinction starburst SEDs. In each galaxy SED category (early-type, Sbc, Scd+Irr, and starburst), the faint-end slopes grow shallower with increasing redshift; in the highest redshift bin (0.4 < z < 0.5), α ∼ − 0.5 and –1.3 for early types and starbursts, respectively. The steepness of α at lower redshifts could be qualitatively explained by LF evolution, or by large numbers of faint dwarf galaxies, perhaps of low surface brightness, that are not detected at higher redshifts.

We use high-resolution (2048² zones) 2D hydrodynamic simulations to study the formation of spiral substructure in the gaseous disk of a galaxy. The obtained gaseous response is driven by a self-consistent nonaxisymmetric potential obtained from an imposed spiral mass distribution. We highlight the importance of ultraharmonic resonances in generating these features. The temporal evolution of the system is followed with the parallel ZEUS-MP code, and we follow the steepening of perturbations induced by the spiral potential until large-scale shocks emerge. These shocks exhibit bifurcations that protrude from the gaseous arms and continue to steepen until new shocks are formed. When the contribution from the spiral potential relative to the axisymmetric background is increased from our default value, spurs protrude from the main arms after several revolutions of the gaseous disk. Such spurs overlap on top of the aforementioned shocks. These results support the hypothesis that a complicated gaseous response can coexist with an orderly spiral potential term, in the sense that the underlying background potential can be smooth yet drive a gaseous response that is far more spatially complex.

Spectral and photometric observations of nearby galaxies show a correlation between the strength of their mid-IR aromatic features, attributed to PAH molecules, and their metal abundances, leading to a deficiency of these features in low-metallicity galaxies. In this paper we suggest that the observed correlation represents a trend of PAH abundance with galactic age, reflecting the delayed injection of carbon dust into the ISM by AGB stars in the final post-AGB phase of their evolution. AGB stars are the primary sources of PAHs and carbon dust in galaxies, and recycle their ejecta back to the ISM after only a few hundred million years of evolution on the main sequence. In contrast, more massive stars that explode as Type II supernovae inject their metals and dust almost instantaneously after their formation. We first determined the PAH abundance in galaxies by constructing detailed models of UV-to-radio SEDs of galaxies that estimate the contribution of dust in PAH-free H II regions, and of PAHs and dust in photodissociation regions, to the IR emission. All model components, the galaxies' stellar content, the properties of their H II regions, and their ionizing and nonionizing radiation fields and dust abundances, are constrained by their observed multiwavelength spectra. After determining the PAH and dust abundances in 35 nearby galaxies using our SED model, we use a chemical evolution model to show that the delayed injection of carbon dust by AGB stars provides a natural explanation for the dependence of the PAH content in galaxies on metallicity. We also show that larger dust particles giving rise to the far-IR emission follow a distinct evolutionary trend closely related to the injection of dust by massive stars into the ISM.

We combine 2MASS data and Spitzer archival data to study the emission in mid-infrared passbands (1.2-24 μm) from a sample of 18 elliptical galaxies. In general the surface brightness distributions resemble de Vaucouleurs profiles, indicating that most of the emission arises from the photospheres or circumstellar regions of red giant stars. The spectral energy distribution peaks near the H band at 1.6 μm. The half-light or effective radius has a pronounced minimum near the K band (2.15 μm) with a second, less consistent minimum in the 24 μm passband. All sample-averaged radial color profiles ⟨ λ_i − λ_j⟩ , where λ_i < λ_j (and j≠ 24 μm), have positive slopes within about twice the (K-band) effective radius. Evidently this variation arises because of an increase in stellar metallicity toward the galactic cores. Color profiles ⟨ K − j⟩ all have positive slopes, particularly when j = 5.8 μm, although no obvious absorption feature is observed in spectra of elliptical galaxies near 5.8 μm. This, and the minimum in R_e, suggests that the K band may be anomalously luminous in metal-rich stars in galaxy cores. Unusual radial color profiles involving the 24 μm passband may suggest that some 24 μm emission comes from interstellar not circumstellar dust grains.

We have imaged in CO(2-1) the molecular gas in NGC 1275 (Perseus A), the cD galaxy at the center of the Perseus Cluster, at a spatial resolution of ~1 kpc over a central region of radius ~10 kpc. Per A is known to contain ~1.3 × 10¹⁰M_☉ of molecular gas, which has been proposed to be captured from mergers with or ram pressure stripping of gas-rich galaxies, or accreted from a X-ray cooling flow. The molecular gas detected in our image has a total mass of ~4 × 10⁹M_☉, and for the first time can be seen to be concentrated in three radial filaments with lengths ranging from at least 1.1 to 2.4 kpc, all lying in the east-west directions spanning the center of the galaxy to radii of ~8 kpc. The eastern and outer western filaments exhibit larger blueshifted velocities with decreasing radii, whereas the inner western filament spans the systemic velocity of the galaxy. The molecular gas shows no signature of orbital motion, and is therefore unlikely to have been captured from gas-rich galaxies. Instead, we are able to reproduce the observed kinematics of the two outer filaments as free fall in the gravitational potential of Per A, as would be expected if they originate from a X-ray cooling flow. Indeed, all three filaments lie between two prominent X-ray cavities carved out by radio jets from Per A, and closely resemble the spatial distribution of the coolest X-ray gas in the cluster core. The inferred mass deposition rate into the two outermost filaments alone is roughly 75 M_☉ yr ⁻¹. This cooling flow can provide a nearly continuous supply of molecular gas to fuel the active nucleus in Per A.

We have obtained deep images in the near-infrared J and K filters of four fields in the Sculptor group spiral galaxy NGC 55 with the ESO VLT and ISAAC camera. For 40 long-period Cepheid variables in these fields, which were recently discovered by Pietrzyński et al., we have determined mean J and K magnitudes from observations at two epochs, and derived distance moduli from the observed period-luminosity (PL) relations in these bands. Using these values together with the previously measured distance moduli in the optical V and I bands, we have determined a total mean reddening of the NGC 55 Cepheids of E(B − V) = 0.127 ± 0.019 mag, which is mostly produced inside NGC 55 itself. For the true distance modulus of the galaxy, our multiwavelength analysis yields a value of 26.434 ± 0.037 mag (random error), corresponding to a distance of 1.94 ± 0.03 Mpc. This value is tied to an adopted true Large Magellanic Cloud (LMC) distance modulus of 18.50 mag. The systematic uncertainty of our derived Cepheid distance to NGC 55 (apart from the uncertainty on the adopted LMC distance) is ±4%, with the main contribution likely to come from the effect of blending of some of the Cepheids with unresolved companion stars. The distance of NGC 55 derived from our multiwavelength Cepheid analysis agrees within the errors with the distance of NGC 300, strengthening the case for a physical association of these two Sculptor group galaxies.

The chemical abundances of neon and sulfur for 25 planetary nebulae (PNe) in the Magellanic Clouds are presented. These abundances have been derived using mainly infrared data from the Spitzer Space Telescope. The implications for the chemical evolution of these elements are discussed. A comparison with similarly obtained abundances of Galactic PNe and H II regions and Magellanic Cloud H II regions is also given. The average neon abundances are 6.0 × 10⁻⁵ and 2.7 × 10⁻⁵ for the PNe in the Large and Small Magellanic Clouds, respectively. These are ~1/3 and 1/6 of the average abundances of Galactic planetary nebulae to which we compare. The average sulfur abundances for the LMC and SMC are, respectively, 2.7 × 10⁻⁶ and 1.0 × 10⁻⁶. The Ne/S ratio (23.5) is on average higher than the ratio found in Galactic PNe (16), but the range of values in both data sets is similar for most of the objects. The neon abundances found in PNe and H II regions agree with each other. It is possible that a few (3-4) of the PNe in the sample have experienced some neon enrichment, but for two of these objects the high Ne/S ratio can be explained by their very low sulfur abundances. The neon and sulfur abundances derived in this paper are also compared to previously published abundances using optical data and photoionization models.

Ionization front instabilities have long been of interest for their suspected role in a variety of phenomena in the Galaxy, from the formation of bright rims and "elephant trunks" in nebulae to triggered star formation in molecular clouds. Numerical treatments of these instabilities have historically been limited in both dimensionality and input physics, leaving important questions about their true evolution unanswered. We present the first three-dimensional radiation hydrodynamical calculations of both R-type (rarefied) and D-type (dense) ionization front instabilities in Galactic environments (i.e., solar-metallicity gas). Consistent with linear stability analyses of planar D-type fronts, our models exhibit many short-wavelength perturbations that grow at early times and later evolve into fewer large-wavelength structures. The simulations demonstrate that both self-consistent radiative transfer and three-dimensional flow introduce significant morphological differences to unstable modes when compared to earlier two-dimensional approximate models. We find that the amplitude of the instabilities in the nonlinear regime is primarily determined by the efficiency of cooling within the shocked neutral shell. Strong radiative cooling leads to long, extended structures with pronounced clumping, while weaker cooling leads to saturated modes that devolve into turbulent flows. These results suggest that expanding H II regions may either promote or provide turbulent support against the formation of later generations of stars, with potential consequences for star formation rates in the Galaxy today.

The high- and intermediate-velocity interstellar clouds (HVCs/IVCs) are tracers of energetic processes in and around the Milky Way. Clouds with near-solar metallicity about 1 kpc above the disk trace the circulation of material between disk and halo (the Galactic fountain). The Magellanic Stream consists of gas tidally extracted from the SMC, tracing the dark matter potential of the Milky Way. Several other HVCs have low metallicity and appear to trace the continuing accretion of infalling intergalactic gas. These assertions are supported by the metallicities (0.1 to 1 solar) measured for about 10 clouds in the past decade. Direct measurements of distances to HVCs have remained elusive, however. In this paper we present four new distance brackets, using VLT observations of interstellar Ca II H and K absorption toward distant Galactic halo stars. We derive distance brackets of 5.0 to 11.7 kpc for the Cohen Stream (likely to be an infalling low-metallicity cloud), 9.8 to 15.1 kpc for Complex GCP (also known as the Smith Cloud or HVC 40–15+100 and with still unknown origin), 1.0 to 2.7 kpc for an IVC that appears associated with the return flow of the fountain in the Perseus arm, and 1.8 to 3.8 kpc for cloud g1, which appears to be in the outflow phase of the fountain. Our measurements further demonstrate that the Milky Way is accreting substantial amounts of gaseous material, which influences the Galaxy's current and future dynamical and chemical evolution.

We present a detailed abundance analysis based on high-resolution and high signal-to-noise spectra of eight extremely metal-poor (EMP) stars with [ Fe/H ] ≲ − 3.5 dex, four of which are new. Only stars with 4900 K < T_eff < 5650 K are included. Two stars of the eight are outliers in each of several abundance ratios. The most metal-poor star in this sample, HE 1424–0241, has [ Fe/H ] ∼ − 4 dex and is thus among the most metal-poor stars known in the Galaxy. It has highly anomalous abundance ratios unlike those of any other known EMP giant, with very low Si, Ca, and Ti relative to Fe, and enhanced Mn and Co, again relative to Fe. Only (low) upper limits for C and N can be derived from the nondetection of the CH and NH molecular bands. HE 0132-2429, another sample star, has excesses of N and Sc with respect to Fe. The strong outliers in abundance ratios among the Fe-peak elements in these C-normal stars, not found at somewhat higher metallicities ([ Fe/H ] ∼ − 3 dex), are definitely real. They suggest that at such low metallicities we are beginning to see the anticipated and long sought stochastic effects of individual supernova events contributing to the Fe-peak material within a single star. With spectra reaching well into the near-UV we are able to probe the behavior of copper abundances in such extreme EMP stars. A detailed comparison of the results of the analysis procedures adopted by our 0Z project compared to those of the First Stars VLT Large Project finds a systematic difference for [ Fe/H ] of ~0.3 dex, our values always being higher.

The currently accepted paradigm for star formation assumes that field stars are born in clusters. These are not formed in isolation but in stellar complexes born out of giant molecular clouds. In the Galactic disk, molecular clouds have distinctive orbits, which, as they disappear after star formation is complete, may seed the Galactic disk with families of young clusters. These families gradually disperse to become individual clusters and, eventually, field populations. We investigate the existence of dynamical families of open clusters in the solar neighborhood using both age- and volume-limited samples from WEBDA in the framework of scan statistics. Our analysis indicates that a significant number of known young clusters organize in groups when age, spatial distribution, and kinematics are taken into account simultaneously. We find compelling statistical evidence for the presence of at least five dynamical families of young open clusters in the Milky Way disk associated to the underlying spiral structure. The young cluster population seems to be dominated by families of 10-20 objects; they are short-lived and the likely progenitors of classical moving groups, and stellar streams. Available observational data suggests that 50%-80% of newly formed open clusters dissolve within 20 Myr of formation to become field population. The overall age distribution of open clusters shows a steep decline, dN/dτ ∝ τ^β, with β = − 3.6 ± 0.5 for clusters younger than 100 Myr, although it could be dependent on the local conditions as it ranges from β = − 1.0 ± 0.2 in the direction of Puppis to β = − 2.8 ± 0.5 in Norma. Due to the high rate of destruction among young clusters, any cluster-related coherent substructure must be younger than about 30 Myr unless it is the result of dynamically induced corotation resonances within the Galactic disk or minor mergers. The characteristic timescale for stars to become part of the field stellar populations is 10-20 Myr.

We present the first detection of complex aldehydes and isomers in three typical molecular clouds located within 200 pc of the center of our Galaxy. We find very large abundances of these complex organic molecules (COMs) in the Central Molecular Zone (CMZ), which we attribute to the ejection of COMs from grain mantles by shocks. The relative abundances of the different COMs with respect to that of CH₃OH are strikingly similar for the three sources, which are located in very different environments in the CMZ. The similar relative abundances point toward a unique grain mantle composition in the CMZ. Studying the Galactic center clouds and objects in the Galactic disk having large abundances of COMs, we find that more saturated molecules are more abundant than the nonsaturated ones. We also find differences between the relative abundance between COMs in the CMZ and the Galactic disk, suggesting different chemical histories of the grain mantles between the two regions in the Galaxy for the complex aldehydes. Different possibilities for the grain chemistry on the icy mantles in the GC clouds are briefly discussed. Cosmic rays can play an important role in the grain chemistry. With these new detections, the molecular clouds in the Galactic center appear to be one of the best laboratories for studying the formation of COMs in the Galaxy.

We report the first fully sampled maps of the distribution of interstellar CO₂ ices, H₂O ices, and total hydrogen nuclei, as inferred from the 9.7 μm silicate feature, toward the star-forming region Cepheus A East with the IRS instrument on board the Spitzer Space Telescope. We find that the column density distributions for these solid state features all peak at, and are distributed around, the location of HW2, the protostar believed to power one of the outflows observed in this star-forming region. A correlation between the column density distributions of CO₂ and water ice with that of total hydrogen indicates that the solid state features we mapped mostly arise from the same molecular clumps along the probed sight lines. We therefore derive average CO₂ ice and water ice abundances with respect to the total hydrogen column density of X(CO₂)_ice ∼ 1.9 × 10⁻⁵ and X(H₂O)_ice ∼ 7.5 × 10⁻⁵. Within errors, the abundances for both ices are relatively constant over the mapped region exhibiting both ice absorptions. The fraction of CO₂ ice with respect to H₂O ice is also relatively constant at a value of 22% over that mapped region. A clear triple-peaked structure is seen in the CO₂ ice profiles. Fits to those profiles using current laboratory ice analogs suggest the presence of both a low-temperature polar ice mixture and a high-temperature methanol-rich ice mixture along the probed sight lines. Our results further indicate that thermal processing of these ices occurred throughout the sampled region.

We have detected the high-excitation lines of carbon-chain molecules such as C₄H₂ (J = 10_0,10–9_0,9), C₄H (N = 9–8, F₁, F₂), l-C₃H₂ (4_1,3-3_1,2), and CH₃CCH (J = 5–4, K = 2) toward a low-mass star-forming region, L1527. In particular, the F₁ line of C₄H is as strong as 1.7 K (T_MB). The rotational temperature of C₄H₂ is determined to be 12.3 ± 0.8 K, which is higher than that in TMC-1 (3.8 K). Furthermore, the column density of C₄H₂ is derived to be about 1/4 of that in TMC-1, indicating that carbon-chain molecules are abundant in L1527 for a star-forming region. Small mapping observations show that the C₄H, C₄H₂, and c-C₃H₂ emissions are distributed from the infalling envelope to the inner part. Furthermore, we have detected the lines of C₅H, HC₇N, and HC₉N in the 20 GHz region. Since the carbon-chain molecules are generally deficient in star-forming cores, the above results cannot simply be explained by the existing chemical models. The following hypothesis is proposed. If the timescale of the prestellar collapse in L1527 were shorter than those of the other star-forming cores, the carbon-chain molecules could survive in the central part of the core. In addition, regeneration processes of the carbon-chain molecules due to star formation activities would play an important role. Evaporation of CH₄ from the grain mantles would drive the regeneration processes. The present observations show new chemistry in a warm and dense region near the protostars, which is named "warm carbon-chain chemistry (WCCC)."

Radiative torques, due to the absorption and scattering of starlight, are thought to play a major role in the alignment of grains with the interstellar magnetic field. The absorption of radiation also gives rise to recoil torques, associated with the photoelectric effect and photodesorption. The recoil torques are much more difficult to model and compute than the direct radiative torque. Here, we consider the relatively simple case of a spheroidal grain. Given our best estimates for the photoelectric yield and other relevant grain physical properties, we find that the recoil torques contribute at the ≈10% level or less compared with the direct radiative torque. We recommend that the recoil torques not be included in models of radiation-driven grain alignment at this time. However, additional experimental characterization of the surface properties and photoelectric yield for submicron grains is needed to better quantify the magnitude of these torques.

G331.5–0.1 in the Norma spiral arm is one of the most luminous and extended cores of a giant molecular cloud (GMC), containing at least six massive and dense dust condensations. Here we report the discovery, from observations of several submillimeter molecular lines that were made using the Atacama Submillimeter Telescope (ASTE) and the Atacama Pathfinder Experiment Telescope (APEX), of an unresolved, extremely high velocity molecular outflow toward the brightest and most massive dust condensation. The outflow is massive and energetic (flow mass of ~55 M_☉; momentum of ~2.4 × 10³M_☉ km s⁻¹; kinetic energy of ~1.4 × 10⁴⁸ ergs). These values are characteristic of flows driven by young massive stellar objects with L_bol ∼ 1 × 10⁵L_☉. We also report the detection, using the Australia Telescope Compact Array (ATCA), of a compact radio continuum source that is located at the center of the outflow and therefore likely to be its driving energy source. It has an spectral index between 4.8 and 8.6 GHz of 1.1 ± 0.2, suggesting that it might correspond to a collimated jet.

To study how the structure of the envelopes of young stellar objects (YSOs) evolve, we carried out a deep JHK'-band imaging survey of the M17 star-forming region using a near-infrared camera and the Infrared Camera and Spectrograph with adaptive optics mounted on the Subaru Telescope. In this survey, we found 51 dark silhouettes against bright near-infrared nebula emissions as background lights. They are regarded as envelopes associated with YSOs due to their size and association with the YSOs. We derived size, morphology, extinction, and mass for each silhouette envelope. The average mass of the envelopes was ~0.02 M_☉, and the radius ranged from 1.5'' to 2.8'' (2250-4200 AU). We compared the properties of envelopes with the properties of the central YSO derived from color-color and color-magnitude diagrams. The radius and the mass of the envelope were found to decrease with the decrease in infrared excess, consistent with the widely accepted view in which the envelope accretes onto the central star/disk system during its evolution from Class I to Class II. However, the envelope still exists around the Class II object for which accretion is barely discernible, and it may have escaped detection due to its dimness. The envelope associated with Class II objects may not accrete onto the star or the disk, but it may dissipate in the future.

In this paper we present the results of a systematic investigation of an entire population of predominately starless dust cores within a single molecular cloud, the Pipe Nebula. Analysis of extinction data shows the cores to be dense objects characterized by a narrow range of density with a median value of n(H₂) = 7 × 10³. The nonthermal velocity dispersions measured in molecular emission lines are found to be subsonic for the large majority of the cores and show no correlation with core mass (or size). Thermal pressure is found to be the dominate source of internal gas pressure and support for most of the core population. The total internal pressures of the cores are found to be roughly independent of core mass over the entire (0.2-20 M_☉) range of the core mass function (CMF) indicating that the cores are in pressure equilibrium with an external source of pressure. This external pressure is most likely provided by the weight of the surrounding molecular cloud. Most of the cores appear to be pressure confined, gravitationally unbound entities whose fundamental physical properties are determined by only a few factors, which include self-gravity, gas temperature, and the simple requirement of pressure equilibrium with the surrounding environment. The entire core population is found to be characterized by a single critical Bonnor-Ebert mass of approximately 2 M_☉. This mass coincides with the characteristic mass of the Pipe CMF suggesting that the CMF (and ultimately the stellar IMF) has its origin in the physical process of thermal fragmentation in a pressurized medium.

Velocity shifts and differential broadening of radio recombination lines are used to estimate the densities and velocities of the ionized gas in several hypercompact and ultracompact H II regions. These small H II regions are thought to be at their earliest evolutionary phase and associated with the youngest massive stars. The observations suggest that these H II regions are characterized by high densities, supersonic flows, and steep density gradients, consistent with accretion and outflows that would be associated with the formation of massive stars.

We constrain blast wave parameters and the circumburst media of a subsample of 10 BeppoSAX gamma-ray bursts (GRBs). For this sample we derive the values of the injected electron energy distribution index, p, and the density structure index of the circumburst medium, k, from simultaneous spectral fits to their X-ray, optical, and NIR afterglow data. The spectral fits have been done in count space and include the effects of metallicity, and are compared with the previously reported optical and X-ray temporal behavior. Using the blast wave model and some assumptions which include on-axis viewing and standard jet structure, constant blast wave energy, and no evolution of the microphysical parameters, we find a mean value of p for the sample as a whole of 2.04^{+ 0.02}_−0.03. A statistical analysis of the distribution demonstrates that the p-values in this sample are inconsistent with a single universal value for p at the 3 σ level or greater, which has significant implications for particle acceleration models. This approach provides us with a measured distribution of circumburst density structures rather than considering only the cases of k = 0 (homogeneous) and k = 2 (windlike). We find five GRBs for which k can be well constrained, and in four of these cases the circumburst medium is clearly windlike. The fifth source has a value of 0 ⩽ k⩽ 1, consistent with a homogeneous circumburst medium.

The optical-UV component in GRB 060218 is assumed to be due to optically thick cyclotron emission. The key aspect of this model is the high temperature of the absorbing electrons. The heat input derives from nuclei accelerated in semirelativistic internal shocks, like in ordinary gamma-ray bursts. Coulomb collisions transfer part of that energy to electrons. Inverse Compton cooling on the X-ray photons leads to electron temperatures around ~100 keV. Such a high brightness temperature for the optical-UV emission implies an emitting area roughly equal to that of the thermal X-ray component. This suggests a model in which the radio, optical-UV, and thermal X-ray emission are closely related. Although the optical-UV and thermal X-ray emission are two separate spectral components, it is argued that they both come from the photosphere of a quasi-spherical, continuous outflow, whose interaction with the circumstellar medium gives rise to the radio emission. The properties of GRB 060218, as measured in the comoving frame, are similar to those of ordinary gamma-ray bursts; i.e., the main difference is the much lower value of the bulk Lorentz factor in GRB 060218. The cyclotron absorption implies a magnetic field in rough equipartition with the matter energy density in the outflow. Hence, the magnetic field could have a dynamically important role, possibly with a magnetar as the central engine.

We report on observations of a gamma-ray burst (GRB 061126) with an extremely bright (R ≈ 12 mag at peak) early-time optical afterglow. The optical afterglow is already fading as a power law 22 s after the trigger, with no detectable prompt contribution in our first exposure, which was coincident with a large prompt-emission gamma-ray pulse. The optical-infrared photometric SED is an excellent fit to a power law, but it exhibits a moderate red-to-blue evolution in the spectral index at about 500 s after the burst. This color change is contemporaneous with a switch from a relatively fast decay to slower decay. The rapidly decaying early afterglow is broadly consistent with synchrotron emission from a reverse shock, but a bright forward-shock component predicted by the intermediate- to late-time X-ray observations under the assumptions of standard afterglow models is not observed. Indeed, despite its remarkable early-time brightness, this burst would qualify as a dark burst at later times on the basis of its nearly flat optical-to-X-ray spectral index. Our photometric SED provides no evidence of host galaxy extinction, requiring either large quantities of gray dust in the host system (at redshift 1.1588 ± 0.0006, based on our late-time Keck spectroscopy) or separate physical origins for the X-ray and optical afterglows.

A series of numerical simulations of magnetorotational core-collapse supernovae are carried out. Dipole-like configurations which are offset northward are assumed for the initially strong magnetic fields, along with rapid differential rotations. The aim of our study is to investigate the effects of the offset magnetic field on magnetar kicks and on supernova dynamics. Note that we study a regime where the proto-neutron star formed after collapse has a large magnetic field strength approaching that of a "magnetar," a highly magnetized slowly rotating neutron star. As a result, equatorially asymmetric explosions occur with the formation of the bipolar jets. We find that the jets are fast and light in the north and slow and heavy in the south for rapid cases, while they are fast and heavy in the north and slow and light in the south for slow-rotation cases. The resulting magnetar kick velocities are ~300-1000 km s⁻¹. We find that the acceleration is mainly due to the magnetic pressure, while the somewhat weaker magnetic tension works in the opposite direction, due to the stronger magnetic field in the northern hemisphere. Note that observations of magnetar proper motions are very scarce; our results supply a prediction for future observations. Namely, magnetars possibly have large kick velocities, several hundred km s⁻¹, as ordinary neutron stars do, and in extreme cases they could have kick velocities up to 1000 km s⁻¹. In each model, the formed protomagnetar is a slow rotator with a rotational period of more than 10 ms. It is also found that, in rapid-rotation models, the final configuration of the magnetic field in the protomagnetar is a collimated dipole-like field pinched by the torus of toroidal field lines, whereas in the protomagnetar produced in the slow-rotation model the poloidal field is totally dominant.

The observed samples of supernovae (SNe) and double compact objects (DCOs) provide several critical constraints on population synthesis models. The parameters of these models must be carefully chosen to reproduce, among other factors, (1) the formation rates of double neutron star (NS-NS) binaries and white dwarf-neutron star (WD-NS) binaries, estimated from binary samples, and (2) the Type II and Ib/c SN rates. Even allowing for extremely conservative accounting of the uncertainties in observational and theoretical predictions, we find that only a few plausible population synthesis models (roughly 9%) are consistent with DCO and SN rates empirically determined from observations. As a proof of concept, we describe the information that can be extracted about population synthesis models given these observational tests, including surprisingly good agreement with the neutron star kick distributions inferred from pulsar proper-motion measurements. In the present study, we find that the current observational constraints favor kicks described by a single Maxwellian with a characteristic velocity of about 350 km s⁻¹ (i.e., at maximum likelihood; kick velocities between 100 and 700 km s⁻¹ remain within the 90% confidence interval of unimodal distributions), mass-loss fractions during nonconservative but stable mass transfer episodes of about 90%, and common envelope parameters of about 0.15-0.5. Finally, we use the subset of astrophysically consistent models to predict the rates at which black hole-neutron star (BH-NS) and NS-NS binaries merge in the Milky Way and the nearby universe, assuming that Milky Way-like galaxies dominate. Inevitably, the resulting probability distributions for merger rates depend on our assumed priors for the population model input parameters. In this study we adopt relatively conservative priors (flat) for all model parameters covering a rather wide range of values. However, as we gain confidence in our knowledge of these inputs, the range of merger rates consistent with our knowledge should shift and narrow.

The interaction of accretion disks with the magnetospheres of young stars can produce X-winds and funnel flows. With the assumption of axial symmetry and steady state flow, the problem can be formulated in terms of quantities that are conserved along streamlines, such as the Bernoulli integral (BI), plus a partial differential equation (PDE), called the Grad-Shafranov equation (GSE), that governs the distribution of streamlines in the meridional plane. The GSE plus BI yields a PDE of mixed type, elliptic before critical surfaces where the flow speed equals certain characteristic wave speeds are crossed and hyperbolic afterward. The computational difficulties are exacerbated by the locations of the critical surfaces not being known in advance. To overcome these obstacles, we consider a variational principle by which the GSE can be attacked by extremizing an action integral, with all other conserved quantities of the problem explicitly included as part of the overall formulation. To simplify actual applications we adopt the cold limit of a negligibly small ratio of the sound speed to the speed of Keplerian rotation in the disk where the X-wind is launched. We also ignore the obstructing effects of any magnetic fields that might thread a disk approximated to be infinitesimally thin. We then introduce trial functions with adjustable coefficients to minimize the variations that give the GSE. We tabulate the resulting coefficients so that other workers can have analytic forms to reconstruct X-wind solutions for various astronomical, cosmochemical, and meteoritical applications.

Gravitationally redshifted absorption lines from highly ionized iron have been previously identified in the burst spectra of the neutron star in EXO 0748–676. To repeat this detection we obtained a long, nearly 600 ks observation of the source with XMM-Newton in 2003. The spectral features seen in the burst spectra from the initial data are not reproduced in the burst spectra from this new data. In this paper we present the spectra from the 2003 observations and discuss the sensitivity of the absorption structure to changes in the photospheric conditions.

We present near-infrared linear spectropolarimetry of a sample of persistent X-ray binaries, Sco X-1, Cyg X-2, and GRS 1915+105. The slopes of the spectra are shallower than what is expected from a standard steady state accretion disk, and can be explained if the near-infrared flux contains a contribution from an optically thin jet. For the neutron star systems, Sco X-1 and Cyg X-2, the polarization levels at 2.4 μm are 1.3% ± 0.10% and 5.4% ± 0.7% , respectively, which is greater than the polarization level at 1.65 μm. This cannot be explained by interstellar polarization or electron scattering in the anisotropic environment of the accretion flow. We propose that the most likely explanation is that this is the polarimetric signature of synchrotron emission arising from close to the base of the jets in these systems. In the black hole system GRS 1915+105 the observed polarization, although high (5.0% ± 1.2% at 2.4 μm), may be consistent with interstellar polarization. For Sco X-1 the position angle of the radio jet on the sky is approximately perpendicular to the near-infrared position angle (electric vector), suggesting that the magnetic field is aligned with the jet. These observations may be a first step toward probing the ordering, alignment, and variability of the outflow magnetic field in a region closer to the central accreting object than is observed in the radio band.

The transient X-ray binary pulsar A0535+262 was observed with Suzaku on 2005 September 14 when the source was in the declining phase of the August-September minor outburst. The ~103 s X-ray pulse profile was strongly energy dependent, with a double-peaked profile in the soft X-ray energy band (<3 keV) and a single-peaked smooth profile in hard X-rays. The width of the primary dip is found to increase with energy. The broadband energy spectrum of the pulsar is well described with a negative and positive power law with exponential (NPEX) continuum model, along with a blackbody component for soft excess. A weak iron Kα emission line with an equivalent width ~25 eV was detected in the source spectrum. The blackbody component is found to be pulsating over the pulse phase, implying that the accretion column and/or the inner edge of the accretion disk may be the possible emission site of the soft excess in A0535+262. The higher value of the column density is believed to be the cause of the secondary dip in the soft X-ray energy band. The iron line equivalent width is found to be constant (within errors) over the pulse phase. However, a sinusoidal type of flux variation of the iron emission line, in phase with the hard X-ray flux, suggests that the inner accretion disk is the possible emission region of the iron fluorescence line.

We have used a model of magnetic accretion to investigate the accretion flows of magnetic cataclysmic variables (mCVs). Numerical simulations demonstrate that four types of flow are possible: disks, streams, rings, and propellers. The fundamental observable determining the accretion flow, for a given mass ratio, is the spin-to-orbital-period ratio of the system. If intermediate polars (IPs) are accreting at their equilibrium spin rates, then for a mass ratio of 0.5, those with P_spin/P_orb≲ 0.1 will be disklike, those with 0.1≲ P_spin/P_orb≲ 0.6 will be streamlike, and those with P_spin/P_orb ∼ 0.6 will be ringlike. The spin-to-orbital-period ratio at which the systems transition between these flow types increases as the mass ratio of the stellar components decreases. For the first time we present evolutionary tracks of mCVs, which make it possible to investigate how their accretion flow changes with time. As systems evolve to shorter orbital periods and smaller mass ratios, in order to maintain spin equilibrium their spin-to-orbital-period ratio will generally increase. As a result, the relative occurrence of ringlike flows will increase, and the occurrence of disklike flows will decrease, at short orbital periods. The growing number of systems observed at high spin-to-orbital-period ratios with orbital periods below 2 hr and the observational evidence for ringlike accretion in EX Hya are fully consistent with this picture.

We present phase-resolved low-resolution infrared spectra of the polar EF Eridani obtained over a period of 2 yr with SpeX on the IRTF. The spectra, covering the wavelength range 0.8 μ m ⩽ λ ⩽ 2.4 μ m , are dominated by cyclotron emission at all phases. We use a "constant lambda" prescription to attempt to model the changing cyclotron features seen in the spectra. A single cyclotron emission component with B≃ 12.6 MG and a plasma temperature of kT≃ 5.0 keV does a reasonable job in matching the features seen in the H and K bands, but fails to completely reproduce the morphology shortward of 1.6 μm. We find that a two-component model, where both components have similar properties but their contributions differ with viewing geometry, provides an excellent fit to the data. We discuss the implications of our models and compare them with previously published results. In addition, we show that a cyclotron model with similar properties to those used for modeling the infrared spectra, but with a field strength of B = 115 MG, can explain the GALEX observations of EF Eri.

We present a comprehensive analysis of the far-ultraviolet spectra of five DB white dwarfs spanning the effective temperature range between 14,700 and 20,800 K. The FUSE line analysis shows that carbon features, previously observed in several hot DB stars at or above 22,000 K, are present in the two coolest (GD 408 and GD 378) and in the hottest (G270–124) target. The observed carbon abundances range from log N(C)/N(He) ∼ − 6.9 to ∼ − 8.8. In addition, four of the five objects display photospheric lines of silicon. Other elements such as oxygen, iron, and sulfur are also observed in some objects. The variations of the abundances of heavy elements as a function of effective temperature in DB stars are discussed in terms of a competition between a stellar wind, gravitational settling, accretion from interstellar (and circumstellar) matter, and convective dredge-up.

We report the detection of a substellar companion orbiting the intermediate-mass giant star 11 Com (G8 III). Precise Doppler measurements of the star from Xinglong Station and Okayama Astrophysical Observatory (OAO) reveal Keplerian velocity variations with an orbital period of 326.03 ± 0.32 days, a semiamplitude of 302.8 ± 2.6 m s⁻¹, and an eccentricity of 0.231 ± 0.005. Adopting a stellar mass of 2.7 ± 0.3 M_☉, the minimum mass of the companion is 19.4 ± 1.5 M_J, well above the deuterium-burning limit, and the semimajor axis is 1.29 ± 0.05 AU. This is the first result from a joint planet-search program between China and Japan aimed at revealing the statistics of substellar companions around intermediate-mass giants. 11 Com b emerged from 300 targets of the planet-search program at OAO. The program's current detection rate of brown dwarf candidates seems to be comparable to the rate of such detections around solar-type stars with orbital separations of ≲3 AU.

We describe Spitzer MIPS observations of the double cluster, h and χ Persei, covering a ~0.6 deg² area surrounding the cores of both clusters. The data are combined with IRAC and 2MASS data to investigate ~616 sources from 1.25-24 μm. We use the long-baseline K_s − [ 24] color to identify two populations with IR excess indicative of circumstellar material: Be stars with 24 μm excess from optically thin free-free emission, and 17 fainter sources (J ∼ 14–15) with [24] excess consistent with a circumstellar disk. The frequency of IR excess for the fainter sources increases from 4.5 to 24 μm. The IR excess is likely due to debris from the planet formation process. The wavelength-dependent behavior is consistent with an inside-out clearing of circumstellar disks. A comparison of the 24 μm excess population in h and χ Per sources with results for other clusters shows that 24 μm emission from debris disks "rises" from 5 to 10 Myr, peaks at ~10-15 Myr, and then "falls" from ~15-20 Myr to 1 Gyr.

Opportunities for extraction of distances and temperatures from eclipsing binary light curves that are in standard flux units, as opposed to traditional arbitrary units, are examined. Benefits include (1) distance becomes an ordinary solution parameter with a standard error, (2) temperatures of both stars may be derivable in favorable circumstances, and (3) semidetached and overcontact binaries suffer no loss of distance accuracy, vis-à-vis well-detached binaries. Flux calibrations enter only for the observations, while theoretical fluxes are naturally in standard units, so confrontation of theory and observation is direct, and semiempirical quantities based on color-temperature relations are not needed. The monolithic process, called direct distance estimation (DDE), also saves time and effort by avoiding separate distance estimation steps and should lead to routine distance measurements for large numbers of eclipsing binaries (EBs). Discussions compare DDE with traditional EB distance estimation, which has been restricted to well detached binaries in most publications. Aspect dependence of spectroscopic or color temperature, as affected by tides and irradiation, is treated rigorously. A temperature-distance theorem that specifies requirements for finding temperatures and distance from EB light and velocity curves is checked by several kinds of simulations. Demonstration solutions are carried out for the overcontact binary AW UMa and the semidetached binary RZ Cnc, with discussions of temperature and distance results. Although AW UMa has many observational and structural oddities, its DDE and Hipparcos distances agree. Six DDE distances for RZ Cnc, done in several ways and for three photometric bands, fall well within the 1 σ range of its Hipparcos distance. Interactions among calibrative errors, parameter values, and fitting discrepancies are discussed, as is proper weighting.

We present multiple epochs of Hα spectroscopy for 47 members of the open cluster NGC 3766 to investigate the long-term variability of its Be stars. Sixteen of the stars in this sample are Be stars, including one new discovery. Of these, we observe an unprecedented 11 Be stars that undergo disk appearances and/or near disappearances in our Hα spectra, making this the most variable population of Be stars known to date. NGC 3766 is therefore an excellent location to study the formation mechanism of Be star disks. From blue optical spectra of 38 cluster members and existing Strömgren photometry of the cluster, we also measure rotational velocities, effective temperatures, and polar surface gravities to investigate the physical and evolutionary factors that may contribute to the Be phenomenon. Our analysis also provides improvements to the reddening and distance of NGC 3766, and we find E(B − V) = 0.22 ± 0.03 and (V − M_V)₀ = 11.6 ± 0.2, respectively. The Be stars are not associated with a particular stage of main-sequence evolution, but they are a population of rapidly rotating stars with a velocity distribution generally consistent with rotation at 70%-80% of the critical velocity, although systematic effects probably underestimate the true rotational velocities, so that the rotation is much closer to critical. Our measurements of the changing disk sizes are consistent with the idea that transitory, nonradial pulsations contribute to the formation of these highly variable disks.

We present results from a study of the orbits of eclipsing binary stars (EBs) in the Magellanic Clouds. The samples comprise 4510 EBs found in the Large Magellanic Cloud (LMC) by the MACHO project, 2474 LMC EBs found by the OGLE-II project (of which 1182 are also in the MACHO sample), 1380 in the Small Magellanic Cloud (SMC) found by the MACHO project, and 1317 SMC EBs found by the OGLE-II project (of which 677 are also in the MACHO sample); we also consider the EROS sample of 79 EBs in the bar of the LMC. Statistics of the phase differences between primary and secondary minima allow us to infer the statistics of orbital eccentricities within these samples. We confirm the well-known absence of eccentric orbit in close binary stars. We also find evidence for rapid circularization in longer period systems when one member evolves beyond the main sequence.

We demonstrate that microlensing can be used for detecting planets in binary stellar systems. This is possible because in the geometry of planetary binary systems, in which the planet orbits one of the binary components and the other binary star is located at a large distance, both planet and secondary companion produce perturbations in a common region around the planet-hosting binary star, and thus the signatures of both planet and binary companion can be detected in the light curves of high-magnification lensing events. We find that identifying planets in binary systems is optimized when the secondary is located within a certain range that depends on the type of the planet. The proposed method can detect planets with masses down to one-tenth of the Jupiter mass in binaries with separations ≲100 AU. These ranges of planet masses and binary separations are not covered by other methods, and thus microlensing would be able to make the planetary binary sample richer.

We study how uncertainties in the rate coefficients of chemical reactions in the RATE 06 database affect abundances and column densities of key molecules in protoplanetary disks. We randomly varied the gas-phase reaction rates within their uncertainty limits and calculated the time-dependent abundances and column densities using a gas-grain chemical model and a flaring steady state disk model. We find that key species can be separated into two distinct groups according to the sensitivity of their column densities to the rate uncertainties. The first group includes CO, C⁺, H⁺₃, H₂O, NH₃, N₂H⁺, and HCNH⁺. For these species the column densities are not very sensitive to the rate uncertainties, but the abundances in specific regions are. The second group includes CS, CO₂, HCO⁺, H₂CO, C₂H, CN, HCN, HNC, and other, more complex species, for which high abundances and abundance uncertainties coexist in the same disk region, leading to larger scatters in column densities. However, even for complex and heavy molecules, the dispersion in their column densities is not more than a factor of ~4. We perform a sensitivity analysis of the computed abundances to rate uncertainties and identify those reactions with the most problematic rate coefficients. We conclude that the rate coefficients of about a hundred chemical reactions need to be determined more accurately in order to greatly improve the reliability of modern astrochemical models. This improvement should be an ultimate goal of future laboratory studies and theoretical investigations.

In several previous articles it has been demonstrated that a quasi-linear description of cosmic-ray transport perpendicular to a mean magnetic field is not appropriate. On the other hand, recently derived nonlinear theories for perpendicular transport are based on several ad hoc assumptions and are difficult to apply due to mathematical problems. In this article we present an alternative formulation for describing perpendicular scattering in turbulent magnetic systems. It is also shown that off-diagonal elements of the spatial diffusion tensor can be so calculated. By combining a quasi-linear formulation for field line random walk with a compound transport model, we derive a semi-quasi-linear formula for calculating the perpendicular diffusion coefficient. By applying this theory onto isotropic turbulence, we demonstrate that test-particle simulations (from Giacalone & Jokipii) can be reproduced analytically. For limiting cases as regards the fraction of the perpendicular to the parallel diffusion coefficient, a simple analytic formula for the perpendicular diffusion coefficients as a function of the parallel diffusion coefficient can be derived.

Recent studies have stressed the importance of solar energetic particle (SEP) transport under disturbed interplanetary conditions, including the case of detection inside a closed interplanetary magnetic loop ejected by a preceding solar event. In this case, particles might be observed to arrive from the far leg of the loop, thus arriving at the detector while traveling sunward. We perform numerical simulations of the focused transport of SEPs along Archimedean spiral and magnetic loop configurations. For loop configurations, we consider injection along either the near leg or the far leg of the loop, either with or without compression at the leading edge. We show that there are specific anisotropy signatures of transport in a closed magnetic loop configuration. SEPs traveling sunward cannot have a high, sustained anisotropy due to the effect of inverse focusing. As an example, the relativistic SEP event of 2003 October 28 exhibited unusual directional distributions, with an early peak of particle flow ≈120° and a main peak ≈80° from the radial direction. However, quantitative fitting of data from the Spaceship Earth network of polar neutron monitors indicates that injection along the far leg of an interplanetary loop is not a good description; our analysis strongly favors transport from the Sun to the Earth over a short path length of ~1 AU.

In this study we compile for the first time comprehensive data sets of solar and stellar flare parameters, including flare peak temperatures T_p, flare peak volume emission measures EM_p, and flare durations τ_f from both solar and stellar data, as well as flare length scales L from solar data. Key results are that both the solar and stellar data are consistent with a common scaling law of EM_p ∝ T^4.7_p, but the stellar flares exhibit ≈250 times higher emission measures (at the same flare peak temperature). For solar flares we observe also systematic trends for the flare length scale L(T_p) ∝ T^0.9_p and the flare duration τ_F(T_p) ∝ T^0.9_p as a function of the flare peak temperature. Using the theoretical RTV scaling law and the fractal volume scaling observed for solar flares, i.e., V(L) ∝ L^2.4, we predict a scaling law of EM_p ∝ T^4.3_p, which is consistent with observations, and a scaling law for electron densities in flare loops, n_p ∝ T²_p/L ∝ T^1.1_p. The RTV-predicted electron densities were also found to be consistent with densities inferred from total emission measures, n_p = (EM_p/q_VV)^1/2, using volume filling factors of q_V = 0.03–0.08 constrained by fractal dimensions measured in solar flares. Solar and stellar flares are expected to have similar electron densities for equal flare peak temperatures T_p, but the higher emission measures of detected stellar flares most likely represent a selection bias of larger flare volumes and higher volume filling factors, due to low detector sensitivity at higher temperatures. Our results affect also the determination of radiative and conductive cooling times, thermal energies, and frequency distributions of solar and stellar flare energies.

In the present work we analyze an extensive active region spectrum observed by the SUMER instrument on board SOHO with the aim of determining the thermal structure of the emitting plasma. We found that the plasma is made of three distinct, isothermal components, whose physical properties are similar to coronal hole, quiet-Sun, and active region plasmas. The temperatures of the coronal hole-like and quiet-Sun-like plasmas are in excellent agreement with previous measurements obtained outside active regions. We also used a DEM diagnostic technique to check the robustness of our results and found that the DEM curves are compatible with the presence of three distinct nearly isothermal plasmas if the individual DEM measurements are smoothed over a small temperature interval. Larger intervals lead the resulting DEM curves to a more multithermal behavior, raising the question of whether multithermal active region DEM curves available in the literature are real or an artifact of oversmoothing. The results are compared with measurements of the temperature of individual loop structures in the literature and discussed in light of a new picture of the solar corona.

We present high-resolution observations of the complex active region AR 0484 observed close to disk center with the Dunn Solar Telescope. The field of view (FOV) contains several interesting features, including a number of umbral fragments, light bridges of varying width, umbral dots, and dark-cored penumbral filaments. A time sequence of reconstructed G-band images and adaptive optics-corrected UBF filtergrams was analyzed with the goal of comparing observations with recent simulations and models of sunspot fine structure. In an umbral fragment in which the field strength is relatively weak, we find a large number of umbral dots. We were able to resolve dark substructure within bright umbral dots that in some cases resembles the dark lanes recently predicted by magnetohydrodynamic (MHD) simulations. Umbral dot substructure is also clearly revealed in images of spectral line parameters. We compare line parameters for dark-cored penumbral filaments, dark lanes observed in light bridges, and the dark umbral dot substructure. We find evidence that all of these structures are elevated above the formation height of the continuum. We observe dynamic proper motion of umbral dots, including motion along a narrow lane that occurs within an umbral fragment and shows similarity to the proper motion observed in narrow light bridges. Furthermore, we study the temporal variation of spectral line parameters such as the integrated line absorption of the temperature-sensitive Fe I λ5434 line. The computed power maps show features closely related to sunspot fine structure.

We use numerical simulations to investigate the production of dust trails by asteroid disruption events. Our work shows that asteroid trails evolve into pairs of dust bands over time. Coherent trails typically survive several tens of kyr before evolving into complete bands after ~1 Myr. The transition timescale depends sensitively on the location of the source breakup event in the main asteroid belt. Bands develop more efficiently from sources in the middle/outer belt than in the inner belt, which may not produce observable pairs of bands at all. The infrared structures produced by recent disruption events (<1 Myr) are characterized by a complicated and changing set of incomplete arcs and cusps. Their geometry depends both on the observer's position and on the source's location in terms of heliocentric distance and inclination to the ecliptic. We postulate that the broad orphan trails named C and D by Sykes in 1988 might have been produced by the formation of the Datura asteroid family 450 ± 50 kyr ago. Additional work will be needed to test this link.

Ca,Al-rich inclusions (CAIs) are believed to have formed by evaporation, condensation, and melting of the pre-existing solids during the earliest stages of the solar system evolution. Most CAIs in unmetamorphosed chondrites contain detectable excesses of ²⁶Mg(²⁶Mg*), a decay product of the short-lived radionuclide ²⁶Al (T_1/2 ∼ 730,000 yr), that correspond to an initial ²⁶Al/²⁷Al ratio of ~(4–7) × 10⁻⁵. It is suggested that ²⁶Al was injected into the protosolar molecular cloud or protoplanetary disk by a nearby core-collapse supernova (SN Type II) and uniformly distributed in the solar system; CAI formation started shortly after injection of ²⁶Al and lasted less than 20,000 yr . Here we show that CAIs from the metal-rich carbonaceous chondrites Acfer 214 (CH) and Isheyevo (CH/CB-like) have a bimodal distribution of ²⁶Mg*. Most CAIs composed of grossite (CaAl₄O₇), hibonite (CaAl₁₂O₁₉), Al-rich pyroxene, perovskite (CaTiO₃), and gehlenitic melilite (Ca₂Al₂SiO₇-Ca₂MgSi₂O₇) show either unresolvable or small ²⁶Mg* corresponding to an initial ²⁶Al/²⁷Al ratio of ~4 × 10⁻⁷. Some of the grossite-rich CAIs and the less refractory inclusions composed of melilite, spinel (MgAl₂O₄), Al,Ti-pyroxene, and anorthite (CaAl₂Si₂O₈) have large ²⁶Mg* corresponding to the initial ²⁶Al/²⁷Al ratio of ~5 × 10⁻⁵. The ²⁶Al-poor and ²⁶Al-rich CAIs are characterized by ¹⁶O-rich (Δ¹⁷O < − 20‰) compositions typical of CAIs. We suggest that the ²⁶Al-poor and ²⁶Al-rich CAIs represent samples of at least two generations of CAIs formed before and after injection of ²⁶Al into the solar system, respectively. Model yields of ¹⁶O,¹⁷O, and ¹⁸O for SN wind prior to explosion, during explosion, and in total, combined with the observations that both ²⁶Al-poor and ²⁶Al-rich CAIs plot on a three-isotope oxygen diagram (δ¹⁷O vs. δ¹⁸O) along a single line with a slope of ~1 are consistent with injection of ²⁶Al with the SN wind into the protosolar molecular cloud rather with the SN explosion into the disk.

The visible electronic spectrum of the nickel hydride B²Δ_5/2–X²Δ_5/2(1,0) and A²Δ_5/2–X²Δ_5/2(1,0) transitions have been recorded with rotational resolution by intracavity laser absorption spectroscopy. The gas-phase NiH molecules were produced in an electric discharge using a nickel hollow cathode in a pure hydrogen atmosphere at 2-3 torr total pressure. Peak positions for NiH isotopologues are presented for both the transitions.⁵⁸NiH and ⁶⁰NiH have been observed in sunspots and attempts to observe NiH in stellar spectra are underway.

The X-ray signature of charge exchange between highly charged L-shell iron ions and neutral gas atoms was studied in the laboratory in order to assess its diagnostic utility. Significant differences with spectra formed by electron-impact excitation were observed. In particular, a strong enhancement was found of the emission corresponding to n ⩾ 4→ n = 2 transitions relative to the n = 3→ n = 2 emission. This enhancement was detectable even with relatively low-resolution X-ray instrumentation (E/Δ E ≈ 10) and may enable future identification of charge exchange as a line-formation mechanism in astrophysical spectra.

The length of the three-dimensional filaments observed in the fourth public data release of the SDSS is measured using the "local skeleton" method. This consists of defining a set of points where the gradient of the smoothed density field is extremal along its isocontours, with some additional constraints on local curvature to probe actual ridges in the galaxy distribution. A good fit to the mean filament length per unit volume, , in the SDSS survey is found to be for 8.2 Mpc ≤ L ≤ 16.4 Mpc, where L is the smoothing length in Mpc. This result, which deviates only slightly, as expected, from the trivial behavior , is in excellent agreement with a ΛCDM cosmology, as long as the matter density parameter remains in the range 0.25 < Ω_matter < 0.4 at the 1 σ confidence level, considering the universe is flat. These measurements, which are in fact dominated by linear dynamics, are not significantly sensitive to observational artifacts such as redshift distortion, edge effects, incompleteness, and biasing. Hence it is argued that the local skeleton is a rather promising and discriminating tool for the analysis of filamentary structures in three-dimensional galaxy surveys.

The origin of the extragalactic gamma-ray background radiation at 1-10 MeV is still unknown. Although the cosmic X-ray background up to a few hundred keV can be accounted for by the sum of active galactic nuclei (AGNs), current models of AGN spectra cannot explain the background spectrum beyond ~1 MeV, because of the thermal exponential cutoff of the electron energy distribution assumed in the models. Here we construct a new spectral model by calculating the Comptonization process, including nonthermal electrons, which are expected to exist in an AGN's hot corona if it is heated by magnetic reconnections. We show that the MeV background spectrum can be explained nicely by our model, when coronal electrons have a nonthermal power-law component whose total energy is a few percent of the thermal component and whose spectral index is dln N_e/dln E_e ≈ − 4. Although in nearby AGN spectra the MeV gamma-ray flux from such a component is below the detection limit of past observations, it might be detected by MeV detectors that are planned for future use. We point out that the nonthermal component's total energy and electron index are similar to those found for electrons accelerated by magnetic reconnections in solar flares and in the Earth's magnetosphere, which supports the reconnection hypothesis, which in turn explains the origin of an AGN's hot coronae.

We report the first direct detection with Spitzer of galaxy filaments. Using Spitzer and ancillary optical data, we have discovered two filamentary structures in the outskirts of the cluster Abell 1763. Both filaments point toward Abell 1770, which lies at the same redshift as Abell 1763 (z = 0.23), at a projected distance of ~13 Mpc. The X-ray cluster emission is elongated along the same direction. Most of the far-infrared emission is powered by star formation. According to the optical spectra, only one of the cluster members is classified as an active galactic nucleus. Star formation is clearly enhanced in galaxies along the filaments: the fraction of starburst galaxies in the filaments is more than twice than that in other cluster regions. We speculate that these filaments are feeding the cluster Abell 1763 by the infall of galaxies and galaxy groups. Evidence for one of these groups is provided by the analysis of galaxy kinematics in the central cluster region.

We present the first deep color-magnitude diagram of the Canes Venatici I (CVn I) dwarf galaxy from observations with the wide-field Large Binocular Camera on the Large Binocular Telescope. Reaching down to the main-sequence turnoff of the oldest stars, it reveals a dichotomy in the stellar populations of CVn I: it harbors an old (≳10 Gyr), metal-poor ([ Fe/H ] ∼ − 2.0), and spatially extended population along with a much younger (~1.4-2.0 Gyr), 0.5 dex more metal-rich, and spatially more concentrated population. These young stars are also offset by pc to the east of the galaxy center. The data suggest that this young population, which represents ~3%-5% of the stellar mass of the galaxy within its half-light radius, should be identified with the kinematically cold stellar component found in a recent spectroscopic survey. CVn I therefore follows the behavior of the other remote MW dwarf spheroidals, which all contain intermediate-age and/or young populations: a complex star formation history is possible in extremely low mass galaxies.

We show that collisions between the outer Galactic H I disk and the leading arms (LAs) of the Magellanic stream (MS) can create giant H I holes and chimney-like structures in the disk. Based on the results of our N-body simulations of the last 2.5 Gyr of evolution of the interaction of the Large and Small Magellanic Clouds (LMC and SMC, respectively) with the Galaxy, we investigate when and where the LAs can pass through the Galactic plane after the MS formation. We then investigate hydrodynamical interaction between LAs and the Galactic H I disk ("the Magellanic impact") by using our new hydrodynamical simulations with somewhat idealized models of the LAs. We find that about 1%-3% of the initial gas mass of the SMC, which consists of the LAs, can pass through the outer part (R = 20–35 kpc) of the Galactic H I disk about 0.2 Gyr ago. We also find that the Magellanic impact can push out some fraction (~1%) of the outer Galactic H I disk to form 1-10 kpc-scale H I holes and chimney-like bridges between the LAs and the disk.

Although the existence of large-scale hot gaseous halos around massive disk galaxies has been theorized for a long time, there is yet very little observational evidence. We report the Chandra and XMM-Newton grating spectral detection of O VII and Ne IX Kα absorption lines along the sight line of 4U 1957+11. The line absorption is consistent with the interstellar medium in origin. Attributing these line absorptions to the hot gas associated with the Galactic disk, we search for the gaseous halo around the Milky Way by comparing this sight line with more distant ones (toward the X-ray binary LMC X-3 and the active galactic nucleus Mrk 421). We find that all the line absorptions along the LMC X-3 and Mrk 421 sight lines are attributable to the hot gas in a thick Galactic disk, as traced by the absorption lines in the spectra of 4U 1957+11 after a Galactic latitude-dependent correction. We constrain the O VII column density through the halo to be N_{O VII} < 5 × 10¹⁵ cm⁻² (95% confidence limit) and conclude that the hot gas contribution to the metal line absorptions, if existing, is negligible.

In four globular clusters (GCs) a nonnegligible fraction of stars can be interpreted only as a very helium-rich population. The evidence comes from the presence of a "blue" main sequence in ω Cen and NGC 2808, and from the very peculiar horizontal-branch morphology in NGC 6441 and NGC 6388. Although a general consensus is emerging on the fact that self-enrichment is a common feature among GCs, the helium content required for these stars is Y≳ 0.35, and it is difficult to understand how it can be produced without any—or, for ω Cen, without a considerable—associated metal enhancement. We examine the possible role of super-AGB stars, and show that they may provide the required high helium. However, the ejecta of the most massive super-AGBs show a global CNO enrichment by a factor of ≃4, due to the dredge-out process occurring at the second dredge-up stage. If these clusters show no evidence for this CNO enrichment, we can rule out that at least the most massive super-AGBs evolve into O-Ne white dwarfs and take part in the formation of the second-generation stars. This latter hypothesis may help to explain the high number of neutron stars present in GCs. The most massive super-AGBs would in fact evolve into electron-capture supernovae. Their envelopes would be easily ejected out of the cluster, but the remnant neutron stars remain in the clusters, thanks to their small supernova natal kicks.

We present the chemical compositions for eight bright giants in the globular cluster NGC 1851. Our analysis reveals large star-to-star abundance variations and correlations of the light elements O, Na, and Al, a feature found in every well-studied globular cluster. However, NGC 1851 also exhibits large star-to-star abundance variations of the s-process elements Zr and La. These s-process elements are correlated with Al and anticorrelated with O. Furthermore, the Zr and La abundances appear to cluster around two distinct values. A recent study revealed a double subgiant branch in NGC 1851. Our data reinforce the notion that there are two stellar populations in NGC 1851 and indicate that this cluster has experienced a complicated formation history with similarities to ω Centauri.

We present observations of the NH₃ (J, K) = (1, 1) and (2, 2) inversion transitions toward the infrared dark cloud G28.34+0.06, using the Very Large Array. Strong NH₃ emission is found to coincide well with the infrared absorption feature in this cloud. The northern region of G28.34+0.06 is dominated by a compact clump (P2) with a high rotation temperature (29 K) and large line width (4.3 km s⁻¹), and is associated with a strong water maser (240 Jy) and a 24 μm point source with far-IR luminosity of 10³L_☉. We infer that P2 has embedded massive protostars although it lies in the 8 μm absorption region. The southern region has filamentary structures. The rotation temperature in the southern region decreases with the increase of the integrated NH₃ intensity, which indicates an absence of strong internal heating in these clumps. In addition, the compact core P1 in the south has small line width (1.2 km s⁻¹) surrounded by extended emission with larger line width (1.8 km s⁻¹), which suggests a dissipation of turbulence in the dense part of the cloud. Thus, we suggest that P1 is at a much earlier evolutionary stage than P2, possibly at a stage that begins to form a cluster with massive stars.

We report the discovery of burst oscillations at 414.7 Hz during a thermonuclear X-ray burst from the low-mass X-ray binary (LMXB) 4U 0614+091 with the Burst Alert Telescope (BAT) on board Swift. In a search of the BAT archive, we found two burst triggers consistent with the position of 4U 0614+091. We searched both bursts for high-frequency timing signatures and found a significant detection at 414.7 Hz during a 5 s interval in the cooling tail of the brighter burst. This result establishes the spin frequency of the neutron star in 4U 0614+091 as ≈415 Hz. The oscillation had an average amplitude (rms) of 14%. These results are consistent with those known for burst oscillations seen in other LMXBs. The inferred ratio of the frequency difference between the twin kHz quasi-periodic oscillations (QPOs) and the spin frequency, Δ ν/ν_s, in this source is strongly inconsistent with either 0.5 or 1 and tends to support the recent suggestions by Yin et al. and Mendez & Belloni that the kHz QPO frequency difference may not have a strong connection to the neutron star spin frequency.

We use high-resolution, three-dimensional hydrodynamic simulations to study the hydrodynamic and gravitational interaction between stellar companions embedded within a differentially rotating common envelope. We evaluate the contributions of the nonaxisymmetric gravitational tides and ram pressure forces to the drag force and, hence, to the dissipation rate and the mass accumulated onto the stellar companion. We find that the gravitational drag dominates the hydrodynamic drag during the inspiral phase, implying that a simple prescription based on a gravitational capture radius significantly underestimates the dissipation rate and overestimates the inspiral decay timescale. Although the mass accretion rate fluctuates significantly, we observe a secular trend leading to an effective rate that is significantly less than the rate based on a gravitational capture radius. We discuss the implications of these results within the context of accretion by compact objects in the common-envelope phase.

It is shown that, for accretion disks, the height scale is a constant whenever hydrostatic equilibrium and the subsonic turbulence regime hold in the disk. In order to have a variable height scale, processes are needed that contribute an extra term to the continuity equation. This contribution makes the viscosity parameter much greater in the outer region and much smaller in the inner region. Under these circumstances, turbulence is the presumable source of viscosity in the disk.

Isolated planetary-mass objects (IPMOs) have masses close to or below the deuterium-burning mass limit (~15 M_Jup)—at the bottom of the stellar initial mass function. We present an exploratory survey for disks in this mass regime, based on a dedicated observing campaign with the Spitzer Space Telescope. Our targets include the full sample of spectroscopically confirmed IPMOs in the σ Orionis cluster, a total of 18 sources. In the mass range 8–20 M_Jup, we identify four objects with >3 σ color excess at a wavelength of 8.0 μm, interpreted as emission from dusty disks. We thus establish that a substantial fraction of IPMOs harbor disks with lifetimes of at least 2-4 Myr (the likely age of the cluster), indicating an origin from core collapse and fragmentation processes. The disk frequency in the IPMO sample is % at 8.0 μm, very similar to what has been found for stars and brown dwarfs (~30%). The object S Ori 70, a candidate 3 M_Jup object in this cluster, shows IRAC colors in excess of the typical values for field T dwarfs (on a 2 σ level), possibly due to disk emission or low gravity. This is a new indication for youth and thus an extremely low mass for S Ori 70.

The amplitude of the Compton-Getting (CG) anisotropy contains the power-law index of the cosmic-ray energy spectrum. Based on this relation and using the Tibet air shower array data, we measure the cosmic-ray spectral index to be –3.03 ± 0.55_stat ± <0.62_syst between 6 and 40 TeV, consistent with –2.7 from direct energy spectrum measurements. Potentially, this CG anisotropy analysis can be utilized to confirm the astrophysical origin of the "knee" against models for nonstandard hadronic interactions in the atmosphere.

We show that candidate contact binary asteroids can be efficiently identified from sparsely sampled photometry taken at phase angles α > 60°. At high phase angle, close/contact binary systems produce distinctive light curves that spend most of the time at maximum or minimum (typically >1 mag apart) brightness with relatively fast transitions between the two. This means that a few (approximately five) sparse observations will suffice to measure the large range of variation and to identify candidate contact binary systems. This finding can be used in the context of all-sky surveys to constrain the fraction of contact binary near-Earth objects. High phase angle light-curve data can also reveal the absolute sense of the spin.

Recent numerical and observational studies contain conflicting reports on the spectrum of magnetohydrodynamic turbulence. In an attempt to clarify the issue we investigate anisotropic incompressible magnetohydrodynamic turbulence with a strong guide field B₀. We perform numerical simulations of the reduced MHD equations in a special setting that allows us to elucidate the transition between weak and strong turbulent regimes. Denote k_||, k_⊥ characteristic field-parallel and field-perpendicular wavenumbers of the fluctuations, and b_λ the fluctuating field at the scale λ ∼ 1/k_⊥. We find that when the critical balance condition, k_||B₀ ∼ k_⊥b_λ, is satisfied, the turbulence is strong, and the energy spectrum is E(k_⊥) ∝ k^−3/2_⊥. As the k_|| width of the spectrum increases, the turbulence rapidly becomes weaker, and in the limit k_||B₀≫ k_⊥b_λ, the spectrum approaches E(k_⊥) ∝ k⁻²_⊥. The observed sensitivity of the spectrum to the balance of linear and nonlinear interactions may explain the conflicting numerical and observational findings where this balance condition is not well controlled.

In past years, measurements of the solar wind plasma have advanced our understanding of MHD turbulence tremendously. At small scales, the solar wind is believed to be ve`xry multifractal with nonlinear interactions causing an intermittent energy dissipation, leading to possible current-sheet structures. In this Letter, we propose a systematic data analysis procedure to examine the existence of current sheets in the solar wind. We show that by studying the integrated distribution function F(θ , ζ) of the angle between two unit magnetic fields and , as well as its ζ-dependence, one can unambiguously identify the existence of current-sheet-like structures in the solar wind. Using this procedure, we analyze magnetic field data from the VHM/FGM instrument on board the spacecraft Ulysses for two periods, one in solar maximum and the other in solar minimum. In both cases, current sheets are clearly inferred. Furthermore, we also outline a procedure that allows the identification of the actual locations of these current sheets. Results from our analysis and the implications of the existence of current sheets in the solar wind are discussed.

The rare phenomenon of ribbon-like hard X-ray (HXR) sources up to 100 keV found in the 2005 May 13 M8.0 flare observed with the Reuven Ramaty High Energy Solar Spectroscopic Imager provides detailed information on the spatial distribution of flare HXR emission. In this Letter, we further investigate the characteristics of HXR emission in this event using imaging spectroscopy, from which we obtain spatially resolved HXR spectral maps during the flare impulsive phase. As a result we found, along a flare ribbon, an anticorrelation relationship between the local HXR flux and the local HXR spectral index. We suggest that this can be regarded as a spatial analog of the well-known temporal soft-hard-soft spectral evolution pattern of the integrated HXR flux. We also found an anticorrelation between the HXR spectral index and the local electric field along the ribbon, which suggests electron acceleration by the electric field during flares.

A massive two-ribbon flare and its source magnetic field region were well captured by the Solar Optical Telescope (SOT) on board Hinode in the Ca II H spectral line and by the Spectro-Polarimeter of SOT, respectively. Using the high-resolution Hinode data sets, we compare the spatial distribution of the local magnetic reconnection rate and the energy release rate along the ribbons with that of G-band kernels that serve as a proxy for the primary energy release. The G-band kernels spatially coincide with the maximum of both modeled quantities, which gives strong support for the reconnection model. We also investigate the magnitude scaling correlation between the ribbon separation speed V_r and magnetic field strength B_n at four 2 minute time bins around the maximum phase of the flare. It is found that V_r is weakly and negatively correlated with B_n. An empirical relation of V_r ∝ B_n^−0.15 is obtained at the flare peak time with an correlation coefficient ~–0.33. The correlation is weaker at other time bins.

The submillimeter-wave spectrum of the MnH and MnD radicals in their ⁷Σ⁺ ground states has been measured in the laboratory using direct absorption techniques. These species were created in the gas phase by the reaction of manganese vapor, produced in a Broida-type oven, with either H₂ or D₂ gas in the presence of a DC discharge. The N = 0 → 1 transition of MnH near 339 GHz was recorded, which consisted of multiple hyperfine components arising from both the manganese and hydrogen nuclear spins. The N = 2 → 3 transition of MnD near 517 GHz was measured as well, but in this case only the manganese hyperfine interactions were resolved. Both data sets were analyzed with a Hund's case b Hamiltonian, and rotational, fine structure, magnetic hyperfine, and electric quadrupole constants have been determined for the two manganese species. An examination of the magnetic hyperfine constants shows that MnH is primarily an ionic species, but has more covalent character than MnF. MnH is a good candidate species for astronomical searches with Herschel, particularly toward material associated with luminous blue variable stars.

A comparison between spectra of carbon nanoparticles in the 16-20 μm range and those of interstellar sources shows that the major emission features attributed to aromatic hydrocarbons in this region are present in laboratory spectra. These features are tentatively identified with bending and wagging modes of polycyclic aromatic hydrocarbons containing aliphatic hydrocarbon side groups. Detailed comparison of laboratory spectra with emission spectra of NGC 7023 shows that variations in the relative intensity of features at 16.4, 16.6, 17.2, 17.4, 17.8, and 18.9 μm arise, in part, from changes in chemical composition. The feature at 18.9 μm appears to be unrelated to that of other bands in laboratory spectra, mimicking the behavior of this feature in NGC 7023. The line widths of these features in laboratory spectra are typically 5-15 cm⁻¹ and are consistent with vibrational de-localization on a timescale of ~10⁻¹² s. This indicates that the emitters of these bands in interstellar sources could contain in excess of 10³ carbon atoms.