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The existence of the three most massive clusters of galaxies observed so far at z > 0.5 is used to constrain the mass density parameter of the universe, Ω, and the amplitude of mass fluctuations, σ₈. We find Ω=0.2_−0.1^+0.3 and σ₈=1.2_−0.4^+0.5 (95%). We show that the existence of even the single most distant cluster at z = 0.83, MS 1054-03, with its large gravitational lensing mass, high temperature, and large velocity dispersion, is sufficient to establish powerful constraints. High-density, Ω = 1 (σ₈ ≃ 0.5-0.6) Gaussian models are ruled out by these data (≲10^-6 probability); the Ω = 1 models predict only ~10^-5 massive clusters at z > 0.65 (~10^-3 at z > 0.5) instead of the one (three) clusters observed.

Among several analytic approximations for the growth of density fluctuations in the expanding universe, Zeldovich-type approximations in Lagrangian coordinate schemes are known to be effective, even in mildly nonlinear regimes. These approximations are very similar in appearance to Padé approximations. We first establish, however, that these two are actually different and independent approximations by using a model of spheroidal mass collapse. Based on this fact, we propose Padé-prescribed Zeldovich-type approximations and demonstrate, within this model, their quantitative and qualitative advantages.

We conduct gravitational microlensing experiments in a galaxy taken from a cosmological N-body simulation. Hypothetical observers measure the optical depth and event rate toward hypothetical LMCs and compare their results with model predictions. Because we control the accuracy and sophistication of the model, we can determine how good it has to be for statistical errors to dominate over systematic ones. Several thousand independent microlensing experiments are performed. When the "best-fit" triaxial model for the mass distribution of the halo is used, the agreement between the measured and predicted optical depths is quite good: by and large, the discrepancies are consistent with statistical fluctuations. If, on the other hand, a spherical model is used, systematic errors dominate. Even with our "best-fit" model, there are a few rare experiments where the deviation between the measured and predicted optical depths cannot be understood in terms of statistical fluctuations. In these experiments there is typically a clump of particles crossing the line of sight to the hypothetical LMC. These clumps can be either gravitationally bound systems or transient phenomena in a galaxy that is still undergoing phase mixing. Substructure of this type, if present in the Galactic distribution of MACHOs, can lead to large systematic errors in the analysis of microlensing experiments. We also describe how hypothetical WIMP and axion detection experiments might be conducted in a simulated N-body galaxy.

The X-ray luminosity-temperature relation for nearby T ≃ 3.5-10 keV clusters is rederived using new ASCA temperatures and ROSAT luminosities. Both quantities are derived by directly excluding the cooling flow regions. This correction results in a greatly reduced scatter in the L_X-T relation; cooling flow clusters are similar to others outside the small cooling flow regions. For a fit of the form L_bol ∝ T^α, we obtain α = 2.64 ± 0.27 (90%) and a residual rms scatter in log L_bol of 0.10. The derived relation can be directly compared to theoretical predictions that do not include radiative cooling. It also provides an accurate reference point for future evolution searches and comparison to cooler clusters. The new temperatures and L_X-T relation together with a newly selected cluster sample are used to update the temperature function at z ~ 0.05. The resulting function is generally higher and flatter than, although within the errors of, the previous estimates by Edge and coworkers and Henry and Arnaud (as rederived by Eke and coworkers). For a qualitative estimate of constraints that the new data place on the density fluctuation spectrum, we apply the Press-Schechter formalism for Ω₀ = 1 and 0.3. For Ω₀ = 1, assuming cluster isothermality, the temperature function implies σ₈ = 0.55 ± 0.03, while taking into account the observed cluster temperature profiles, σ₈ = 0.51 ± 0.03, consistent with the previously derived range. The dependence of σ₈ on Ω₀ is different from the earlier results because of our treatment of the slope of the fluctuation spectrum, n, as a free parameter. For the considered values of Ω₀, n = -(2.0-2.3) ± 0.3, somewhat steeper than that derived from the earlier temperature function data, in agreement with the local slope of the galaxy fluctuation spectrum from the Automatic Plate Measuring Facility (APM) survey, and significantly steeper than the standard cold dark matter prediction.

We report the 0.5-10 keV X-ray image and spectrum of Cl 0016+16, which, at a redshift of 0.541, is one of the most distant clusters of galaxies observed with ASCA. The ASCA X-ray image is well represented by an isothermal β model. The best-fit parameters for the core radius (θ_c) and β are 06 and 0.7, respectively. However, because of ASCA's moderate spatial resolution, these two parameters are strongly correlated. The observed spectrum within a 6' radius is well fitted by a thin thermal emission model with a plasma temperature of 8.0 ± 1.0 keV. The Sunyaev-Zeldovich (SZ) effect in Cl 0016+16 has been observed by Carlstrom et al. using an interferometer and by Birkinshaw et al. using a single-dish radio telescope. Combining the SZ microwave decrement with the plasma density and temperature profiles derived from our X-ray observation, we obtained a value of the Hubble constant of 47 ± 14 km s^-1 Mpc^-1 with the profile parameters obtained by interferometric data, and 52 ± 20 km s^-1 Mpc^-1 with our best-fit shape parameters and single-dish telescope data.

We investigate a one-zone chemophotometric evolution model of disk-disk galaxy mergers in order to clarify whether galaxy mergers with a widely spread merging epoch can reproduce reasonably well the observed small scatter of the color-magnitude (C-M) relation in cluster ellipticals at low and intermediate redshift (z < 1). We consider that merger-progenitor disks begin to consume interstellar gas at a moderate rate from z ~ 5 and then merge to form an elliptical with a secondary starburst at z = z_merge. We find that even if the epoch of galaxy merging is rather extended (0.3 < z_merge < 3.0), the dispersion in the rest-frame U-V color among galaxy mergers is well within the observed one (~0.05 mag at z = 0). We also find that the z_merge is required to be within a certain range to keep the observed C-M relation tight at a given z. For example, the required range of z_merge in galaxy mergers between Sa disks is 1.3 < z_merge < 3.0 for cluster ellipticals at z = 0.895, 0.9 < z_merge < 3.0 for z = 0.55, and 0.3 < z_merge < 3.0 for z = 0. The main reason for the derived small scatter is that younger stellar populations, which are formed during the secondary starburst of galaxy mergers, are formed preferentially from more metal-enriched interstellar gas. This result reinforces Worthey's suggestion of 1996 that the age-metallicity conspiracy, which means that younger stellar populations are preferentially more metal-enriched, can operate to keep the tight C-M relation. These numerical results imply that the observed small scatter in the C-M relation at low and intermediate redshift (z < 1) doesnotnecessarily require the coevality of elliptical galaxies in clusters or their formation at high z, which has been conventionally believed in the classical, passive evolution picture.

We numerically investigate the chemodynamical evolution of the interstellar medium (ISM) in gas-rich disk-disk galaxy mergers in order to explore the origin of fundamental chemical properties of halo ISM observed in elliptical galaxies. There are three main results of this chemodynamical study: (1) Elliptical galaxies formed by gas-rich mergers show steep negative metallicity gradients in the ISM, especially in the outer parts of galaxies. This is because chemical evolution of the ISM in mergers proceeds in such an inhomogeneous way that metal enrichment of the ISM is more efficient in the central part of mergers, as a result of the radial inflow of metal-enriched ISM during dissipative galaxy merging, whereas in the outer part, metal enrichment is less efficient because a larger amount of metal-enriched ISM is tidally stripped away from mergers. This result provides a clue to the origin of gaseous metallicity gradients in elliptical halos recently revealed by ASCA. (2) Because of the inhomogeneous chemical evolution of the ISM in mergers, some merger remnants show a mean gaseous metallicity that is discernibly smaller than the mean stellar one. The degree of difference between the mean stellar and gaseous metallicities in a merger remnant depends on chemical mixing length, galactic mass, and the effectiveness of supernova feedback. (3) Elliptical galaxies formed by multiple mergers are more likely to have metal-poor gaseous halo components and steep gaseous metallicity gradients than those formed by pair mergers. This is principally because a larger amount of less metal enriched ISM can be tidally stripped away more efficiently from galaxies in multiple mergers. These three results demonstrate that dynamical evolution of gas-rich galaxy mergers, in particular tidal stripping of less metal enriched ISM during galaxy merging, greatly determines the chemical evolution of the ISM of galaxy mergers. These results furthermore imply that recent ASCA observational results on the mean and radial chemical properties of halo ISM in elliptical galaxies can be understood in terms of the chemodynamical evolution of gas-rich galaxy mergers.

Making the usual assumption that the relatively cold matter within the central engine of an active galactic nucleus (or galactic black hole candidate) is in the form of a relativistic accretion disk, we compute the composite spectrum of the original disk plus a primary X-ray power-law source illuminating it from above, as well as the reflected emission from the disk. All special and general relativistic effects on both infalling photons and outgoing photons are considered in a Schwarzschild geometry. The strength, shape, and broadening of the reflected spectrum depend on the direction of the X-ray source relative to the disk and the observer's viewing angle. The reflected photons extract energy and angular momentum from the relativistically rotating accretion disk and are beamed in the direction of the disk velocity. The reflection hump could essentially disappear if viewed far from the symmetry axis because the X-ray photons are affected by gravity both approaching and leaving the disk. This may produce a difference between X-ray spectra for Seyfert 1 and Seyfert 2 galaxies. For a given observation angle, the reflection hump is most sensitive to the inclination of the source relative to the accretion disk. Thus the spectral shape may also shed light on the location of the primary X-ray source, which is probably either in a jet or in a corona; however, additional computations involving distributed sources will be necessary before detailed comparisons with observations are feasible.

We present the first results from the ISO-IRAS Faint Galaxy Survey (IIFGS), a program designed to obtain ISO observations of the most distant and luminous galaxies in the IRAS Faint Source Survey by filling short gaps in the ISO observing schedule with pairs of 12 μm ISOCAM and 90 μm ISOPHOT observations. As of 1997 October, over 500 sources have been observed, with an ISOCAM detection rate over 80%, covering over 1.25 deg² of sky to an 11.5 μm point-source completeness limit of approximately 1.0 mJy (corresponding to a ~10 σ detection sensitivity). Observations are presented for nine sources detected by ISOPHOT and ISOCAM early in the survey for which we have ground-based G- and I-band images and optical spectroscopy. The ground-based data confirm that the IIFGS strategy efficiently detects moderate-redshift (z = 0.11-0.38 for this small sample) strong emission line galaxies with L_{60 μm} ≳ 10¹¹L_☉; one of our sample has L_{60 μm} > 10¹²L_☉ (H₀ = 75 km s^-1 Mpc^-1, Ω = 1). The infrared-optical spectral energy distributions are comparable to those of nearby luminous infrared galaxies, which span the range from pure starburst (e.g., Arp 220) to infrared QSO (Mrk 231). Two of the systems show signs of strong interaction, and four show active galactic nucleus (AGN)-like excitation; one of the AGNs, F15390+6038, which shows a high excitation Seyfert 2 spectrum, has an unusually warm far- to mid-infrared color and may be an obscured QSO. The IIFGS sample is one of the largest and deepest samples of infrared-luminous galaxies available, promising to be a rich sample for studying infrared-luminous galaxies up to z ~ 1 and for understanding the evolution of infrared galaxies and the star formation rate in the universe.

Recent work by Pringle and by Maloney, Begelman, & Pringle has shown that geometrically thin, optically thick, accretion disks are unstable to warping driven by radiation torque from the central source. This work was confined to isothermal (i.e., surface density Σ ∝ R^-3/2) disks. In this paper we generalize the study of radiation-driven warping to include general power-law surface density distributions, Σ ∝ R^-δ. We consider the range from δ = 3/2 (the isothermal case) to δ = -3/2, which corresponds to a radiation-pressure-supported disk; this spans the range of surface density distributions likely to be found in real astrophysical disks. In all cases there are an infinite number of zero-crossing solutions (i.e., solutions that cross the equator), which are the physically relevant modes if the outer boundary of the disk is required to lie in a specified plane. However, unlike the isothermal disk, which is the degenerate case, the frequency eigenvalues for δ ≠ 3/2 are all distinct. In all cases the location of the zero moves outward from the steady state (pure precession) value with increasing growth rate; thus, there is a critical minimum size for unstable disks. Modes with zeros at smaller radii are damped. The critical radius and the steady state precession rate depend only weakly on δ. An additional analytic solution has been found for δ = 1. The case δ = 1 divides the solutions into two qualitatively different regimes. For δ ≥ 1, the fastest growing modes have maximum warp amplitude, β_max, close to the disk outer edge, and the ratio of β_max to the warp amplitude at the disk inner edge, β₀, is ≫1. For δ < 1, β_max/β₀ ≃ 1, and the warp maximum steadily approaches the origin as δ decreases. This implies that nonlinear effects must be important if the warp extends to the disk inner edge for δ ≥ 1, but for δ < 1 nonlinearity will be important only if the warp amplitude is large at the origin. Because of this qualitative difference in the shapes of the warps, the effects of shadowing of the central source by the warp will also be very different in the two regimes of δ. This has important implications for radiation-driven warping in X-ray binaries, for which the value of δ characterizing the disk is likely to be less than unity. In real accretion disks the outer boundary condition is likely to be different from the zero-crossing condition that we have assumed. In accretion disks around massive black holes in active galactic nuclei, the disk will probably become optically thin before the outer disk boundary is reached, whereas in X-ray binaries there will be an outer disk region (outside the circularization radius) in which the inflow velocity is zero but angular momentum is still transported. We show that in both these cases the solutions are similar to the zero-crossing eigenfunctions.

We discuss near-infrared spectra of 20 interacting galaxies from the Arp Catalog and analyze the properties of similar galaxies for which only optical spectra are available. We find excellent agreement between the types of activity determined from the infrared and optical, demonstrating that obscuration does not seriously bias the optical results. None of the 20 galaxies show infrared spectral characteristics that differ from expectations for isolated galaxies; the very strong shock-excited lines seen in a few interacting systems must be uncommon. Most of the galaxies in our infrared sample are the sites of starbursts that appear to have had durations of 1 to 5 Myr and to be observed 3 to 10 Myr after the peak star-forming episode. Four of the 20 galaxies have LINER or composite starburst/LINER spectra that are likely to arise from shocks due to supernovae in late phase starbursts. In the full interacting galaxy sample, there is a substantial excess of Seyfert 2 nuclei, supporting previous indications that this type of activity tends to occur in interacting host galaxies.

An object discovered during an infrared survey of the field near the quasar B2 0149+33 has an emission line at 2.25 μm that we interpret as Hα at a redshift of 2.43. The K-band image shows two compact components 10 kpc apart, surrounded by more extended emission over ~20 kpc. The Hα emission appears to be extended over ~15 kpc (2'') in a coarsely sampled (08 pixel^-1) image. The star formation rate may be as high as 250-1000 M_☉ yr^-1, depending on the extinction. Alternatively, the line may be powered by an active nucleus, although the probability of serendipitously discovering an AGN in the survey volume is only ~0.02. The increasing number of similar objects reported in the literature indicate that they may be an important, unstudied population in the high-redshift universe.

We present Faint Object Camera (FOC) ultraviolet images of the central 14'' × 14'' of Messier 31 and Messier 32. The hot stellar populations detected in the composite UV spectra of these nearby galaxies are partially resolved into individual stars, and their individual colors and apparent magnitudes are measured. We detect 433 stars in M31 and 138 stars in M32, down to detection limits m_F275W = 25.5 mag and m_F175W = 24.5 mag. We investigate the luminosity functions of the sources, their spatial distribution, their color-magnitude diagrams, and their total integrated far-UV flux. Comparison to IUE and Hopkins Ultraviolet Telescope (HUT) spectrophotometry and WFPC2 stellar photometry indicates consistency at the 0.3 mag level, with possible systematic offsets in the FOC photometry at a level less than this. Further calibrations or observations with the Space Telescope Imaging Spectrograph (STIS) will be necessary to resolve the discrepancies. Our interpretation rests on the assumption that the published FOC on-orbit calibration is correct. Although M32 has a weaker UV upturn than M31, the luminosity functions and color-magnitude diagrams (CMDs) of M31 and M32 are surprisingly similar and are inconsistent with a majority contribution from any of the following: post-asymptotic giant branch (PAGB) stars more massive than 0.56 M_☉ (with or without associated planetary nebulae), main sequence stars, and blue stragglers. Both the luminosity functions and color-magnitude diagrams are consistent with a dominant population of stars that have evolved from the extreme horizontal branch (EHB) along tracks with masses between 0.47 and 0.53 M_☉. These stars are well below the detection limits of our images while on the zero-age EHB but become detectable while in the more luminous (but shorter) AGB-manqué and post-early asymptotic giant branch (PEAGB) phases. The FOC observations require that only very small fractions of the main-sequence populations (2% in M31 and 0.5% in M32) in these two galaxies evolve though the EHB and post-EHB phases, with the remainder evolving through bright PAGB evolution that is so rapid that few if any stars are expected in the small field of view covered by the FOC. A model with a flat EHB star mass distribution reproduces the HUT and IUE spectra of these two galaxies reasonably well, although there is some indication that an additional population of very hot (T_eff > 25,000 K) EHB stars may be needed to reproduce the HUT spectrum of M31 near the Lyman limit and to bring integrated far-UV fluxes of M31 and M32 into agreement with IUE. In addition to the post-EHB population detected in the FOC, we find a minority population (~10%) of brighter stars that populate a region of the CMD that cannot be explained by canonical post-HB evolutionary tracks. The nature of these stars remains open to interpretation. The spatial distributions of the resolved UV-bright stars in both galaxies are more centrally concentrated than the underlying diffuse emission, implying that stellar populations of different ages and/or metallicities might be responsible for each component.

We present and discuss optical continuum images made with the HubbleSpaceTelescope of the radio galaxy 1138-262 at z = 2.2. This object possesses one of the clumpiest optical morphologies of all known high-redshift radio galaxies, consisting of a bright nucleus aligned along the radio axis and a number of smaller components distributed over a region as large as the radio source (130 kpc). The clumps have sizes ranging from 3 to 13 kpc and absolute visual magnitudes between M_V = -21.8 and M_V = -19.4. On the basis of HST and previous observations, we claim that these clumps are star-forming galaxies that will be accreted by the host galaxy of 1138-262. We compare this radio galaxy with other high-redshift objects and with the predictions of current scenarios of galaxy formation.

We report VLBA images of the nucleus of the Seyfert galaxy NGC 1068 at 1.7, 5, and 15 GHz, with resolutions between 3 and 10 mas (0.2-0.7 pc) and a sensitivity of ~10⁶ K at all three frequencies. Our goals are to study the morphology of the radio emission at subparsec resolution and to investigate thermal gas in the putative obscuring disk or torus and in the narrow-line region clouds through free-free absorption of the radio emission. All four known radio components in the central arcsecond (S2, S1, C, and NE, from south to north) have been detected at either 1.7 or 5 GHz, or both. No radio emission was detected at 15 GHz. Component S1 is probably associated with the active nucleus, with radio emission originating from the inner edge of the obscuring torus according to Gallimore et al. Our observed flux densities at 1.7 and 5 GHz are in agreement with their thermal bremsstrahlung emission model, and we find that the nuclear radiation may be strong enough to heat the gas in S1 to the required temperature of ~4 × 10⁶ K. The bremsstrahlung power would be 0.15(f_refl/0.01) times the bolometric luminosity of the nucleus between 10^14.6 and 10^18.4 Hz (where f_refl is the fraction of radiation reflected into our line of sight by the electron-scattering mirror) and so the model is energetically reasonable. We also discuss two other models for S1 that also match the observed radio spectrum: electron scattering by the torus of radio emission from a compact synchrotron self-absorbed source and synchrotron radiation from the torus itself. Components NE and S2 have spectra consistent with optically thin synchrotron emission, without significant absorption. Both of these components are elongated roughly perpendicular to the larger scale radio jet, suggesting that their synchrotron emission is related to transverse shocks in the jet or to bow shocks in the external medium. Component C has a nonthermal spectrum absorbed at low frequency. This absorption is consistent with free-free absorption by plasma with conditions typical of narrow-line region clouds.

Recent near-infrared imaging surveys of the giant molecular cloud L1641 have revealed the existence of small clusters consisting of on the order of 10-50 young stars. While these observations suggest that stars may actually form in a range of stellar densities, the multiplicity of pre-main-sequence stars, along with the emerging evidence for protostellar condensations, indicate that stars may ultimately form through fragmentation of collapsing molecular cloud cores. Previous collapse calculations have shown that prolate cloud models are highly susceptible to fragmentation into protostellar binaries. While these results apply to star formation in an isolated environment, small cluster formation may require starting the collapse from oblate, rather than prolate, shapes. Here we use a smoothed-particle hydrodynamics code to further investigate the gravitational collapse and fragmentation of centrally condensed (Gaussian), oblate molecular clouds with varied thermal energy (α) and axial ratios of 2:1, 4:1, and 8:1. All models start with uniform rotation and ratios of rotational to gravitational energy β ≈ 0.02. Because of the oblate geometry, the clouds first collapse down to their equatorial plane, forming a rotationally unbalanced, intermediate central core that may be spherical, oblate spheroidal, or disklike, depending on the value of α. The evolution proceeds with the central core collapsing radially inward while becoming progressively flatter. During this phase, a rotationally supported, thin pancake forms, which quickly becomes gravitationally unstable to the point of fragmentation. The results indicate that the 2:1 clouds all produced low-order protostellar systems consisting of 2-3 clumps for 0.18 ≤ α ≤ 0.48. Similarly, the 4:1 and 8:1 clouds formed low-order systems of 3-5 clumps for α ≥ 0.37 and α = 0.48, respectively. Formation of small clusters of ~10 protostars occurred only in the 4:1 clouds for α = 0.18 and in the 8:1 clouds for α ≤ 0.37. Thus, increasing the degree of cloud oblateness extends the range of α values for which fragmentation into a larger number of clumps is allowed. Since extreme axial ratios of 4:1 and 8:1 are consistent with the observations, the results may have implications for understanding the formation of the small clusters observed in L1641.

Parallax data from the Hipparcos mission allow the direct distance to open clusters to be compared with the distance inferred from main-sequence (MS) fitting. There are surprising differences between the two distance measurements, indicating either the need for changes in the cluster compositions or reddening, underlying problems with the technique of MS fitting, or systematic errors in the Hipparcos parallaxes at the 1 mas level. We examine the different possibilities, focusing on MS fitting in both metallicity-sensitive B-V and metallicity-insensitive V-I for five well-studied systems (the Hyades, Pleiades, α Per, Praesepe, and Coma Ber). The Hipparcos distances to the Hyades and α Per are within 1 σ of the MS-fitting distance in B-V and V-I, while the Hipparcos distances to Coma Ber and the Pleiades are in disagreement with the MS-fitting distance at more than the 3 σ level. There are two Hipparcos measurements of the distance to Praesepe; one is in good agreement with the MS-fitting distance and the other disagrees at the 2 σ level. The distance estimates from the different colors are in conflict with one another for Coma but in agreement for the Pleiades. Changes in the relative cluster metal abundances, age related effects, helium, and reddening are shown to be unlikely to explain the puzzling behavior of the Pleiades. We present evidence for spatially dependent systematic errors at the 1 mas level in the parallaxes of Pleiades stars. The implications of this result are discussed.

This paper examines the discrepancy between distances to nearby open clusters as determined by parallaxes from Hipparcos compared to traditional main-sequence fitting. The biggest difference is seen for the Pleiades, and our hypothesis is that if the Hipparcos distance to the Pleiades is correct, then similar subluminous zero-age main-sequence (ZAMS) stars should exist elsewhere, including in the immediate solar neighborhood. We examine a color-magnitude diagram of very young and nearby solar-type stars and show that none of them lie below the traditional ZAMS, despite the fact that the Hipparcos Pleiades parallax would place its members 0.3 mag below that ZAMS. We also present analyses and observations of solar-type stars that do lie below the ZAMS, and we show that they are subluminous because of low metallicity and that they have the kinematics of old stars.

We have determined the location of the line-opacity modified Eddington limit for stars in the LMC using the most recent atmosphere models combined with a precise mapping to the H-R diagram through up-to-date stellar evolution calculations. While we find, in agreement with previous studies, that the shape of the modified Eddington limit qualitatively corresponds to the Humphreys-Davidson (HD) limit defined by the most luminous supergiants, the modified limit is actually afullmagnitudehigher than the upper luminosity limit observed for LMC stars. The observed limit is consistent with atmosphere models in which the maximum value of the ratio of the radiation force outward to the gravitational force inward (Y_max) is ~0.9, i.e., the photospheres of stars at the observed luminosity limit are bound. As massive stars evolve, they move to higher, and therefore less stable, values of Y_max, so mass loss, either sporadic or continuous, may halt their natural redward evolution as they approach the observed Y_max ≈ 0.9 limit. We assess the metallicity dependence of this limit. If mass loss does limit the redward evolution of the most luminous stars, and if the value of Y_max corresponding to the luminosity limit in the LMC is universal, then the brightest supergiants of the SMC should be only marginally brighter (0.3 mag) than those of the LMC, in agreement with observations. Moreover, the brightest supergiants in M31 should be 0.75 mag fainter than those in the LMC.

In a study of the velocity dispersion of molecular gas in NH₃ maps of four dense cores, we find that: (1) within the interiors of dense cores, the line widths are roughly constant at a value slightly but significantly higher than a purely thermal line width; and (2) at the edges of the maps of the dense cores, it appears that the line width starts to increase. We suggest that these dense cores are "coherent" in that the nonthermal, turbulent, contributions to the line width are so small that observed velocity dispersion is nearly independent of scale within the cores. In the second paper of this series (Goodman et al.), by analyzing maps of the cores' environments, we find an apparent transition to this coherent regime from a more turbulent one, at about the size scale of a FWHM NH₃ contour, or ~0.1 pc.

Analysis of velocity gradients in dense cores and their environs indicates that the cores appear to spin independently of their surroundings, along an axis not obviously related to their shape. Comparison of gradients implied by the relative velocities of high-density cores in complexes and gradients in the extended low-density gas in these complexes suggests a picture in which the coherent cores behave as (independently spinning) test particles, floating along in a turbulent flow.

An appendix to this paper presents a new algorithm for predicting the intrinsic width of the 18 hyperfine lines in the NH₃ inversion spectrum from a Gaussian fit to the main hyperfine blend and an estimate of the optical depth.

After studying how line width depends on spatial scale in low-mass star-forming regions, we propose that "dense cores" (Myers & Benson 1983) represent an inner scale of a self-similar process that characterizes larger scale molecular clouds.

In the process of coming to this conclusion, we define four distinct types of line width-size relation (Δv∝R^a_i), which have power-law slopes a₁, a₂, a₃, and a₄, as follows: Type 1—multitracer, multicloud intercomparison; Type 2—single-tracer, multicloud intercomparison; Type 3—multitracer study of a single cloud; and Type 4—single-tracer study of a single cloud. Type 1 studies (of which Larson 1981 is the seminal example) are compendia of Type 3 studies which illustrate the range of variation in the line width-size relation from one region to another.

Using new measurements of the OH and C¹⁸O emission emanating from the environs of several of the dense cores studied in NH₃ by Barranco & Goodman (1998; Paper I), we show that line width increases with size outside the cores with a₄ ~ 0.2. On scales larger than those traced by C¹⁸O or OH,¹²CO and ¹³CO observations indicate that a₄increases to ~0.5 (Heyer & Schloerb 1997). By contrast, within the half-power contour of the NH₃ emission from the cores, line width is virtually constant, with a₄ ~ 0. We interpret the correlation between increasing density and decreasing Type 4 power-law slope as a "transition to coherence." Our data indicate that the radius R_coh at which the gas becomes coherent (i.e., a₄ → 0) is of order 0.1 pc in regions forming primarily low-mass stars. The value of the nonthermal line width at which "coherence" is established is always less than but still of order of the thermal line width of H₂. Thus coherent cores are similar to, but not exactly the same as, isothermal balls of gas.

Two other results bolster our proposal that a transition to coherence takes place at ~0.1 pc. First, the OH, C¹⁸O, and NH₃ maps show that the dependence of column density on size is much steeper (N ∝ R^-0.9) inside R_coh than outside of it (N ∝ R^-0.2), which implies that the volume filling factor of coherent cores is much larger than in their surroundings. Second, Larson (1995) has recently found a break in the power law characterizing the clustering of stars in Taurus at 0.04 pc, just inside of R_coh. Larson and we interpret this break in slope as the point at which stellar clustering properties change from being determined by the (fractal) gas distribution (on scales greater than 0.04 pc) to being determined by fragmentation processes within coherent cores (on scales less than 0.04 pc).

We speculate that the transition to coherence takes place when a dissipation threshold for the MHD turbulence that characterizes the larger scale medium is crossed at the critical inner scale R_coh. We suggest that the most likely explanation for this threshold is the marked decline in the coupling of the magnetic field to gas motions due to a decreased ion/neutral ratio in dense, high filling factor gas.

We obtain self-similar solutions that describe the gravitational collapse of nonrotating, isothermal, magnetic molecular cloud cores. We use simplifying assumptions but explicitly include the induction equation, and the semianalytic solutions we derive are the first to account for the effects of ambipolar diffusion and its critical dependence on the magnetic tension force following the formation of a central point mass. Our results demonstrate that, after the protostar first forms, ambipolar diffusion causes the magnetic flux to decouple in a growing region around the center. The decoupled field lines remain approximately stationary and drive a hydromagnetic C-shock that moves outward at a fraction of the speed of sound (typically a few tenths of a kilometer per second), reaching a distance of a few thousand AU at the end of the main accretion phase for a solar-mass star. We also show that, in the absence of field diffusivity, a contracting core will not give rise to a shock if, as is likely to be the case, the inflow speed near the origin is nonzero at the time of point-mass formation. Although the evolution of realistic molecular cloud cores will not be exactly self similar, our results reproduce the main qualitative features found in detailed core-collapse simulations.

We present a numerical simulation of the dynamical collapse of a nonrotating, magnetic molecular cloud core and follow the core's evolution through the formation of a central point mass and its subsequent growth into a 1 M_☉ protostar. The epoch of point-mass formation (PMF) is investigated by a self-consistent extension of previously presented models of core formation and contraction in axisymmetric, self-gravitating, isothermal, magnetically supported interstellar molecular clouds. Prior to PMF, the core is dynamically contracting and is not well approximated by a quasi-static equilibrium model. Ambipolar diffusion, which plays a key role in the early evolution of the core, is unimportant during the dynamical pre-PMF collapse phase. However, the appearance of a central mass, through its effect on the gravitational field in the inner core regions, leads to a "revitalization" of ambipolar diffusion in the weakly ionized gas surrounding the central protostar. This process is so efficient that it leads to a decoupling of the field from the matter and results in an outward-propagating hydromagnetic C-type shock. The existence of an ambipolar diffusion-mediated shock of this type was predicted by Li & McKee, and we find that the basic shock structure given by their analytic model is well reproduced by our more accurate numerical results. Our calculation also demonstrates that ambipolar diffusion, rather than Ohmic diffusivity operating in the innermost core region, is the main field-decoupling mechanism responsible for driving the shock after PMF. The passage of the shock leads to a substantial redistribution, by ambipolar diffusion but possibly also by magnetic interchange, of the mass contained within the magnetic flux tubes in the inner core. In particular, ambipolar diffusion reduces the flux initially threading a collapsing ~1 M_☉ core by a factor ≳10³ by the time this mass accumulates within the inner radius (≃7.3 AU) of our computational grid. This reduction, which occurs primarily during the post-PMF phase of the collapse, represents a significant step toward the resolution of the protostellar magnetic flux problem. Our calculations indicate that a 1 M_☉ protostar forms in ~1.5 × 10⁵ yr for typical cloud parameters. The mass-accretion rate is time dependent, in part because of the C-shock that decelerates the infalling matter as it propagates outward: the accretion rate rises to ≃9.4 M_☉ Myr^-1 early on and decreases to ≃5.6 M_☉ Myr^-1 by the time a solar-mass protostar is formed. The infalling gas disk surrounding the protostar has a mass ~10^-2M_☉ at radii r ≳ 500 AU. A distinguishing prediction of our model is that the rapid ambipolar diffusion after the formation of a protostar should give rise to large (≳1 km s^-1), and potentially measurable, ion-neutral drift speeds on scales r ≲ 200 AU. The main features of our simulation, including the C-shock formation after PMF, are captured by a similarity solution that incorporates the effects of ambipolar diffusion.

Numerical simulations and analytical solutions have established that ambipolar diffusion can reduce the dust-to-gas ratio in magnetically and thermally supercritical cores during the epoch of core formation. We study the effect that this has on the ion chemistry in contracting protostellar cores and present a simplified analytical method that allows one to calculate the ion power-law exponent k (≡ d ln n_i/d ln n_n, where n_i and n_n are the ion and neutral densities, respectively) as a function of core density. We find that, as in earlier numerical simulations, no single value of k can adequately describe the ion abundance for n_n ≲ 10⁹ cm^-3, a result that is contrary to the "canonical" value of k = found in previous static equilibrium chemistry calculations and often used to study the effect of ambipolar diffusion in interstellar clouds. For typical cloud and grain parameters, reduction of the abundance of grains results in k > during the core formation epoch (densities ≲10⁵ cm^-3). As a consequence, observations of the degree of ionization in cores could be used, in principle, to determine whether ambipolar diffusion is responsible for core formation in interstellar molecular clouds. For densities ≫10⁵ cm^-3, k is generally ≪.

The conversion factor, X_CO, between the velocity-integrated CO(1-0) antenna temperature, W(CO), and the H₂ column density, N(H₂), is determined for 32 positions in two translucent high-latitude molecular clouds, MBM 40 and MBM 16. X_CO is calculated using CH observations of the ²Π_1/2F = 1-1 hyperfine transition to infer N(H₂). The latter quantity, divided by W(CO) yields X_CO for various positions across the clouds. We observed 24 positions in MBM 16, and X_CO values were derived in the range (1.6-17.3) × 10²⁰ cm^-2 (K km s^-1)^-1, with a mean value of 7.6 × 10²⁰. Eleven lines of sight were sampled in MBM 40 yielding values of X_CO in the range (0.7-9.7) × 10²⁰, with a mean value of 2.6 × 10²⁰. An inverse relationship between X_CO and W(CO) may exist, suggesting that the variation in X_CO for these two translucent clouds arises from varying CO abundances. This paper also reports the existence of a broad component in the CH spectra observed throughout much of MBM 16. This component possibly originates in the disturbed outer regions of the cloud where the gas is not gravitationally bound to the core of the cloud. It is unclear how sensitive the CO rotational transitions are to this component, but it is likely that the CO/H₂ ratio in this broad-line gas is less than 10^-5. However, if the CH/H₂ ratio is the same for the gas in the extended wings as it is for the typical cloud gas, then up to 40% of the cloud mass could be contained in this difficult to trace molecular component.

The dynamics of molecular clouds is characterized by supersonic random motions in the presence of a magnetic field. We study this situation using numerical solutions of the three-dimensional compressible magnetohydrodynamic (MHD) equations in a regime of highly supersonic random motions. The non-LTE radiative transfer calculations are performed through the complex density and velocity fields obtained as solutions of the MHD equations, and more than 5 × 10⁵ spectra of ¹²CO,¹³CO, and CS are obtained. In this way we build synthetic molecular clouds of 5 and 20 pc diameter, evolved for about one dynamical time from their initial configuration. We use a numerical flow without gravity or external forcing. The flow is super-Alfvénic.

Synthetic data consist of sets of 90 × 90 synthetic spectra with 60 velocity channels, in five molecular transitions: J = 1 → 0 and J = 2 → 1 for ¹²CO and ¹³CO, and J = 1 → 0 for CS. Although we do not consider the effects of stellar radiation, gravity, or mechanical energy input from discrete sources, our models do contain the basic physics of magnetofluid dynamics and non-LTE radiation transfer and are therefore more realistic than previous calculations. As a result, these synthetic maps and spectra bear a remarkable resemblance to the corresponding observations of real clouds.

We report aperture synthesis C¹⁸O J = 1-0 observations of L1551 IRS 5 with a spatial resolution of 28 × 25 using the Nobeyama Millimeter Array. We have detected an emission component centrally condensed around IRS 5, as well as a diffuse component extending in the north-south direction from the centrally condensed component. The centrally condensed component, 2380 × 1050 AU in size, is elongated in the direction perpendicular to the outflow axis, indicating the existence of a flattened circumstellar envelope around L1551 IRS 5. The mass of the centrally condensed component is estimated to be 0.062 M_☉. The position-velocity (P-V) diagrams reveal that the velocity field in the centrally condensed component is composed of infall and slight rotation. The infall velocity in the outer part is equal to the free-fall velocity around a central mass of ~0.1 M_☉, e.g., 0.5 km s^-1 at r = 700 AU, whereas the rotation velocity, 0.24 km s^-1 at the same radius, gets prominent at inner radii with a radial dependence of r^-1. We make up P-V diagrams for the model envelopes with vertical structure, in which the matter falls under the gravity and eventually settles down in Keplerian rotation inside the centrifugal radius, and compare them with the observed P-V diagrams of the centrally condensed component. The main characteristics of the observed P-V diagrams are reproduced by either (1) an envelope with a moderately flattened density distribution, or (2) a spherical envelope with a bipolar cavity whose half-opening angle is about 50°. Detailed comparison of the observed and model P-V diagrams suggests that the C¹⁸O J = 1-0 emission from the outer part of the centrally condensed component is well reproduced with the models with the central mass ~0.15 M_☉ and the mass infall rate ~6 × 10^-6M_☉ yr^-1. However, the higher velocity features of the emission near the star cannot be reproduced unless the central mass is taken to be ~0.5 M_☉. These facts suggest either that the gas pressure and/or magnetic force dilute the effect of the gravity in the outer part of the envelope, or that the velocity structure inside the centrifugal radius deviates significantly from the Keplerian rotation.

We present the results of single-dish observations of CS J = 2-1 and J = 3-2 and interferometric observations of CO J = 1-0 toward the center of the quadrupolar molecular outflow in L723. We have detected a compact CS condensation having a size of 0.04 pc and a mass of 0.55 M_☉ toward the northeastern radio continuum source VLA 2 (AER91 2). The CO outflow also shows the distribution centered at VLA 2. These results suggest that the source VLA 2 is the young stellar object that is powering the conspicuous molecular outflow system. On the other hand, there is no enhancement in the CS intensity or the CO outflow distribution toward the southwestern radio continuum source VLA 1 (AER91 1), indicating that the source VLA 1 does not contribute to the morphology of the quadrupolar outflow in L723.

The CO distribution observed with the interferometer delineates the western edge of the blue lobe and the northeastern edge of the red lobe revealed in the single-dish map, suggesting that the outflow in L723 is a single bipolar outflow with a wide opening angle of 120°-170° rather than two independent outflows. We found signs of interaction between the blueshifted outflow and the dense ambient gas: (1) there is a compact CS clump blueshifted by ~1 km s^-1, the distribution of which shows anticorrelation with the blueshifted CO outflow; (2) both CS and NH₃ spectra show the line broadening toward the blueshifted clump; and (3) there is a temperature enhancement at the boundary of the blueshifted clump of CS emission. It is likely that such interaction with the dense ambient gas has the increased opening angle of the outflow, which accounts for the quadrupolar morphology.

Narrowband images of the Crab Nebula taken with the HubbleSpaceTelescope (HST) WFPC2 show the morphology and ionization structure of the filaments in great detail. At HST resolution, low- and high-ionization emission from filaments in the Crab differ in two complementary respects. First, low-ionization emission is found to be concentrated in very sharp structures, while high-ionization emission is predominantly found in a much more diffuse component. For example, approximately 80% of emission from [O I] λ6300 arises in features with scales of less than 05, while only 10% of [O III] λ5007 emission arises in such compact structures. Second, individual filaments are found to lie along a sequence of ionization structures, ranging from features in which all lines are concentrated in the same compact volume through features with low-ionization cores surrounded by high-ionization envelopes. Hester and coworkers proposed in 1996 that this sequence can be understood as the result of the nonlinear development of magnetic Rayleigh-Taylor (R-T) instabilities along the interface between the Crab synchrotron nebula and swept-up ejecta.

We present photoionization models of cylindrically symmetrical filaments consisting of a quadratic core surrounded by an extended envelope. A good deal of the observed variation in filament structure in the Crab can be matched by varying the assumed density profiles in these models. This implies that variations in the development of R-T instabilities in the Crab account for much of the spectral variation within the remnant. We also present a photoionization model of a uniform, low-density medium, which reasonably matches the extended diffuse component that dominates the high-ionization emission. This envelope model produces strong [O III] but virtually no [O I]. While the He I/Hβ ratio remains fairly constant throughout a range of filament models, this ratio is a factor of 5 lower in the envelope model. We find that the apertures used in ground-based spectroscopy of the Crab generally include emission from several discrete filaments as well as a component of diffuse emission. This places a fundamental limit on what can be inferred reliably from comparison of spectra with one-dimensional photoionization models. Many filament cores coincide with dust extinction features seen in a continuum image of the Crab. We consider one such feature in detail and find that the extinction of about 1.2 mag suggests that the dust-to-gas mass ratio may be an order of magnitude higher than is typical in the interstellar medium.

New infrared images of Cep A East are presented that show two regions of shock-excited line emission from separate bipolar flows. We identify the dominant sources powering the outflows and argue that the results support a multiple outflow model (Narayanan & Walker) as opposed to a quadrupolar outflow scenario. The images include near-infrared broadband ( [2.158 μm], L'' [3.81 μm], and M' [4.67 μm]) and spectral line ([Fe II] emission line at 1.644 μm and H₂ 1-0 S[1] line at 2.122 μm) observations, as well as continuum emission, at 1.644 μm and 2.122 μm. Considering our data and other results, we present a unified, self-consistent picture of the disk and shock structure. The northern emission region appears to be the result of the ablation of a dense molecular clump (coincident with HW 6) in the path of a diverting jet from YSO HW 2 and subsequent multiple bow shocks with prompt entrainment arising from the interaction of the jet with the molecular cloud Cep A-2. The southern line emission region near HW 7 resembles the "artillery shell" bow shocks found in Orion and is most likely a J-type shock caused by a jet from another YSO, possibly HW 3(d)ii.

We present high-sensitivity, high-resolution VLA observations of the W3 complex of H II regions in the 168α recombination lines of hydrogen, carbon, and sulphur. The H 168α line from W3A consists of two components: a broad line (width ~27 km s^-1) and a narrow line (width ~7 km s^-1). The narrow hydrogen and carbon line emissions over W3A, although overlapping, are not entirely coextensive. The carbon line is possibly correlated with the molecular gas near W3A. Stimulated emission is the main mechanism for the narrow hydrogen line emission. The width of the H⁰ line gives an upper limit of ~1000 K for the electron temperature of the partially ionized gas. The electron density ranges from 10 to 80 cm^-3 in the narrow hydrogen line region and from 10 to 60 cm^-3 in the carbon-line region. We determined the electron temperature of the classical H II region W3A from the continuum brightness to be ~9000 K. The rms n_e of this H II region is ~2200 cm^-3, and the true n_e, determined from a pressure-broadened profile of the H 171η (8.6 GHz) line, is ~2 × 10⁴ cm^-3. Using these two values of electron densities, we determine a lower limit to the filling factor (0.01). Such a low value can be interpreted as an effect of density inhomogeneities in the medium.

Profile comparison of the Stokes parameters V and I is a powerful tool for maser data analysis, which provides the first direct methods for unambiguous determination of (1) the maser saturation stage, (2) the amplification optical depth and intrinsic Doppler width of unsaturated masers, and (3) the comparative magnitudes of Zeeman splitting and Doppler line width. Circular polarization recently detected in OH 1720 MHz emission from the Galactic center appears to provide the first direct evidence for maser saturation.

Gamma-ray bursts are mysterious flashes of gamma radiation the properties of which have shown virtually no significant, nondefinitional correlations or bimodalities. The physical processes which relate burst properties to one another could be revealed by finding significant relations. In the past, searches for correlations and bimodalities in the data have been made for a relatively small set of properties. We present the results of a systematic search for statistically significant correlations and bimodalities from a set of 49 burst properties, with particular attention paid to the establishment and inclusion of light-curve characteristics. We have used the database of the 260 bursts in the 1B catalog of BATSE. No unpublished, significant, nondefinitional correlations correlations were found, nor were any original, nonsystematic bimodalities observed. Our method recovered the known correlation between the log of duration and spectral hardness with a probability that the result is a random fluctuation of 9.3 × 10^-5. The previously reported bimodality in the logarithm of duration was recovered.

I describe a new time-domain algorithm for detecting localized structures (bursts), revealing pulse shapes, and generally characterizing intensity variations. The input is raw counting data, in any of three forms: time-tagged photon events (TTE), binned counts, or time-to-spill (TTS) data. The output is the most probable segmentation of the observation into time intervals during which the photon arrival rate is perceptibly constant, i.e., has no statistically significant variations. The idea is not that the source is deemed to have this discontinuous, piecewise constant form, rather that such an approximate and generic model is often useful. Since the analysis is based on Bayesian statistics, I call the resulting structures Bayesian blocks. Unlike most, this method does not stipulate time bins—instead the data determine a piecewise constant representation. Therefore the analysis procedure itself does not impose a lower limit to the timescale on which variability can be detected. Locations, amplitudes, and rise and decay times of pulses within a time series can be estimated independent of any pulse-shape model—but only if they do not overlap too much, as deconvolution is not incorporated. The Bayesian blocks method is demonstrated by analyzing pulse structure in BATSE γ-ray data.

In a previous paper, we have written equations describing steady state, optically thin, advection-dominated accretion onto a Kerr black hole. In this paper, we survey the numerical solutions to these equations. We find that the temperature and density of the gas in the inner part of the accretion flow depend strongly on the black hole spin parameter a. The rate of angular momentum accretion is also shown to depend on a; for a greater than an equilibrium spin parameter a_eq, the black hole is de-spun by the accretion flow. We also investigate the dependence of the flow on the angular momentum transport efficiency α, the advected fraction of the dissipated energy f, and the adiabatic index γ. We find solutions for -1 < a < 1, 10^-4 ≤ α ≤ 0.44, 0.01 ≤ f ≤ 1, and 4/3 < γ < 5/3. For low values of α and f, the inner part of the flow exhibits a pressure maximum and appears similar to equilibrium thick disk solutions.

We study neutrino emission from the remnant of an inspiraling binary neutron star following coalescence. The mass of the merged remnant is likely to exceed the stability limit of a cold, rotating neutron star. However, the angular momentum of the remnant may also approach or even exceed the Kerr limit, J/M² = 1, so that total collapse may not be possible unless some angular momentum is dissipated. We find that neutrino emission is very inefficient in decreasing the angular momentum of these merged objects and may even lead to a small increase in J/M². We illustrate these findings with a post-Newtonian, ellipsoidal model calculation. Simple arguments suggest that the remnant may form a bar mode instability on a timescale similar to or shorter than the neutrino emission timescale, in which case the evolution of the remnant will be dominated by the emission of gravitational waves.

Neutron stars in binary orbit emit gravitational waves and spiral slowly together. During this inspiral, they are expected to have very little vorticity. It is in fact a good approximation to treat the system as having zero vorticity, i.e., as irrotational. Because the orbital period is much shorter than the radiation reaction timescale, it is also an excellent approximation to treat the system as evolving through a sequence of equilibrium states, in each of which the gravitational radiation is neglected. In Newtonian gravity, one can simplify the hydrodynamic equations considerably for an equilibrium irrotational binary by introducing a velocity potential. The equations reduce to a Poisson-like equation for the potential, and a Bernoulli-type integral for the density. We show that a similar simplification can be carried out in general relativity. The resulting equations are much easier to solve than other formulations of the problem.

We present phase-resolved near-infrared broadband photometry of four short-period cataclysmic variables (HU Aqr, WZ Sge, TY Psc, and V592 Cas). Coupled with ultraviolet and optical data obtained from the literature, we have modeled the spectral energy distributions of these four cataclysmic variables, as well as that of the twin of WZ Sge, AL Com. The secondary stars contribute no more than 20%-50% of the near-infrared flux except for the polar HU Aqr, where the secondary contributes ~75% of the near-infrared flux. For the systems located above the orbital period minimum, the temperatures of the secondary stars match those for the expected main-sequence secondary stars. However, our modeling places WZ Sge below the orbital period minimum and shows it containing a secondary star of less than 1700 K—the coldest "star" yet identified.

I am using the ROSAT All-Sky Survey to identify M dwarfs within 25 pc of the Sun that are missing from the Catalogue of Nearby Stars. Selection by X-rays is very efficient for this purpose since the stars found will tend to be young and, hence, have small space motions. It is just such stars that are missing from the Catalogue of Nearby Stars, which largely consists of stars first discovered in proper-motion surveys. In this paper, I present an initial list of 54 M dwarfs, which, by virtue of their photometric parallaxes, should be included in the Catalogue. These preliminary results are also used to estimate that, because of this effect, the amount of mass that is missing from estimates of the local Galactic mass density is 0.003 M_☉ pc^-3. This amount is insufficient to solve the Galactic "missing mass" problem.

We use numerical three-dimensional hydrodynamics to investigate how assumptions about local thermal conditions affect the strength and outcome of nonaxisymmetric instabilities in massive protostellar disks. Building on work presented in earlier papers, we generate two protostellar core models that represent equilibrium states that could form from the axisymmetric collapse of uniformly rotating, singular isothermal spheres. Both models are continuous star/disk systems, in which the star, the disk, the star/disk boundary, and the free disk outer boundary are resolved in three dimensions. The models are distinguished primarily by the temperature distribution in the disk, and both can be considered to represent the same early evolutionary stage of disk development, when the disk is massive but small in radial extent. In the "hot" model, the disk is assumed to have the same entropy per gram as the central isentropic star, giving a Toomre Q-parameter ~2.5 over the disk region. In the "cool" model, the entropy per gram decreases radially outward in the disk, resulting in more realistic, cooler disk temperatures and Q ≈ 1.5. Each of these protostellar star/disk systems is evolved in our three-dimensional hydrodynamics code under two different assumptions about thermal equilibrium in the disk, namely that either the entropy per gram or the temperature remains constant with position in the disk. We refer to these two cases as locally isentropic evolution and locally isothermal evolution, respectively. All four calculations have been run for at least two outer rotation periods of the disk. With either assumption about the thermal equilibrium, one- and two-armed spiral disturbances, which grow in the hot models, saturate at low amplitude (~1%) and do not alter the protostellar core significantly. On the other hand, the cool model is highly unstable to multiple low-order spirals, which induce significant mass and angular momentum transport in a few dynamical times. Under locally isentropic evolution, the star and star/disk boundary in the cool model are unstable to three- and four-armed disturbances and the disk is unstable to a two-armed spiral, but all these modes saturate at moderate nonlinear (a few tens percent) amplitudes after about 1.5 outer rotation periods. The same instabilities occur under locally isothermal evolution; however, the two-armed spiral in the disk grows more vigorously and does not saturate, ultimately disrupting the disk and concentrating material into thin, dense arcs and arclets that approach stellar densities. In both cool model calculations, there is substantial inward transport of mass and outward transport of angular momentum during the growth phase of the two-armed spiral, but the transport rate drops by over an order of magnitude for locally isentropic evolution when the two-armed spiral saturates. It is clear from these calculations that thermal energetics play a critical role in the development of self-gravitating instabilities and that, under conditions of strong cooling, such instabilities can disrupt a disk very early in its development. We compare these calculations with previous work on gravitational instabilities in disks and discuss implications for star and planet formation.

We have analyzed 32 individual graphite grains from the Murchison meteorite for their Mo and/or Zr isotopic compositions by resonant ionization mass spectrometry. Enormous isotopic anomalies were observed in some of these grains for both elements. The data for Zr revealed the largest isotopic anomalies, with ⁹⁶Zr/⁹⁴Zr ratios ranging from 0.074 times to 10 times the solar value. The isotopic data on Mo show one population of graphite grains with close-to-terrestrial Mo composition in all isotopes and five grains with an s-process nucleosynthesis signature, i.e., correlated depletions in the p- and r-process isotopes. For eight grains we were able to measure both Mo and Zr isotopic compositions. Three of these eight graphite grains have s-process isotopic characteristics for both Zr and Mo, which suggests low-mass, thermally pulsed asymptotic giant branch stars as their origin. Four grains are puzzling, since they have nearly normal Mo compositions but significant anomalies in Zr, in particular, large depletions or enhancements in the ⁹⁶Zr/⁹⁴Zr ratio. Two of these grains have extraordinary enrichments in ⁹⁶Zr, with ⁹⁶Zr/⁹⁴Zr ratios 10.4 ± 1.3 and 2.5 ± 0.3 times the solar system value. These enrichments are suggestive of the r-process, implying that these grains condensed from the ejecta of core-collapse supernovae, but these enrichments could also be made by the s-process if the neutron density were unusually high.

Large excesses of ⁴⁴Ca in certain presolar graphite and silicon carbide grains give strong evidence for ⁴⁴Ti production in supernovae. Furthermore, recent detection of the ⁴⁴Ti γ line from the Cas A supernova remnant by the ComptonGammaRayObservatory Compton Telescope shows that radioactive ⁴⁴Ti is produced in supernovae. These make the ⁴⁴Ti abundance an observable diagnostic of supernovae. Through use of a nuclear reaction network, we have systematically varied reaction rates and groups of reaction rates to experimentally identify those that govern ⁴⁴Ti abundance in core-collapse supernova nucleosynthesis. We survey the nuclear-rate dependence by repeated calculations of the identical adiabatic expansion, with peak temperature and density chosen to be 5.5 × 10⁹ K and 10⁷ g cm^-3, respectively, to approximate the conditions in detailed supernova models. We find that, for equal total numbers of neutrons and protons (η = 0),⁴⁴Ti production is most sensitive to the following reaction rates:⁴⁴Ti(α, p)⁴⁷V, α(2α, γ)¹²C,⁴⁴Ti(α, γ)⁴⁸Cr, and ⁴⁵V(p, γ)⁴⁶Cr. We tabulate the most sensitive reactions in order of their importance to the ⁴⁴Ti production near the standard values of currently accepted reaction rates, at both a reduced reaction rate (times 0.01) and an increased reaction rate (times 100) relative to their standard values. Although most reactions retain their importance for η > 0, that of ⁴⁵V(p, γ)⁴⁶Cr drops rapidly for η ≥ 0.0004. Other reactions assume greater significance at greater neutron excess:¹²C(α, γ)¹⁶O,⁴⁰Ca(α, γ)⁴⁴Ti,²⁷Al(α, n)³⁰P,³⁰Si(α, n)³³S. Because many of these rates are unknown experimentally, our results suggest the most important targets for future cross section measurements governing the value of this observable abundance.

We report results of analysis of the ASCA observation of 1993 May 7 of the dipping low-mass X-ray binary (LMXB) source XBT 0748-676 and propose a new explanation of the spectral evolution in dipping in this source. The behavior of the source was very unusual in that, in the band 1-3 keV, dipping extended around most of the orbital cycle with almost no nondip intensity evident, and the depth of dipping reached 100%. At higher energies, e.g., 3-10 keV, the depth of dipping was less than 100%, and there were marked increases in hardness in dipping. We show that the nondip and dip spectra in several intensity bands are well fitted using the same physical model that we have previously shown gives good explanations of several dipping sources, consisting of point-source blackbody emission from the neutron star plus extended Comptonized emission from the accretion disk corona (ADC), with progressive covering of the ADC during dipping. Best-fit values of kT_bb = 1.99 ± 0.16 keV and power-law photon index Γ = 1.70 ± 0.16 are found. The strong excess below 1 keV was well fitted by a Gaussian line at 0.65 keV. In dipping, good fits were obtained by allowing it to be covered by the same progressive covering factor as the extended continuum emission, providing strong evidence that the line originates in the ADC. Our approach of applying the two-component model and explicitly including progressive covering of the Comptonized emission differs radically from the "absorbed + unabsorbed" approach previously used extensively for XBT 0748-676 and similar sources, in which the normalization of the unabsorbed peak in dip spectra is allowed to decrease by a large factor in dipping. This decrease has often been attributed to the effects of electron scattering. By using our two-component model, we show that the unabsorbed component is the uncovered fraction of the Comptonized emission, and in the band 1-10 keV, we do not need to invoke electron scattering to explain dipping.

We present ultra-high-resolution (0.32 km s^-1) spectra obtained with the 3.9 m Anglo-Australian Telescope (AAT) and Ultra-High-Resolution Facility (UHRF) of interstellar Na I D1, Na I D2, Ca II K, K I, and CH absorption toward two high Galactic latitude stars HD 141569 and HD 157841. We have compared our data with 21 cm observations obtained from the Leiden/Dwingeloo H I survey. We derive the velocity structure and column densities of the clouds represented by the various components and identify the clouds with ISM structures seen in the region at other wavelengths. We further derive abundances, linear depletions, and H₂ fractional abundances for these clouds wherever possible. Both stars are located in regions of IRAS 100 μm emission associated with high Galactic latitude molecular clouds (HLCs): HD 141569 lies, in projection, close to MBM 37 and the Lynds dark cloud L134N, whereas HD 157841 is in the vicinity of the MBM 151. Toward HD 141569 we detect two components in our UHRF spectra: a weak, broad b = 4.5 km s^-1 component at -15 km s^-1, seen only in Ca II K absorption, and another component at 0 km s^-1, seen in Na I D1, Na I D2, Ca II K, K I, and CH absorption. The cloud represented by the -15 km s^-1 component is warm and may be located in a region close to the star. The cloud represented by the 0 km s^-1 component has a Ca linear depletion δ(Ca) = 1.4 × 10^-4 and shows evidence for the presence of dust, consistent with strong 100 μm emission seen in this region. The H₂ fractional abundance f(H₂) derived for this cloud is 0.4, which is typically what is observed toward HLCs. We conclude that this 0 km s^-1 cloud is associated with MBM 37 and L134N based on the presence of dust and molecular gas (CH) and good velocity agreement with CO emission from these two clouds. This places HD 141569 beyond MBM 37 and L134N, which are estimated to be at ≈ 110 pc. In the case of the HD 157841 sight line, a total of six components are seen on our UHRF spectra in Na I D1, Na I D2, Ca II K, K I, and CH absorption. Two of these six components are seen only in a single species. The cloud represented by the components at 1.85 km s^-1 has a Ca linear depletion δ(Ca) = 2.8 × 10^-4, indicating the presence of dust. The f(H₂) derived for this cloud is 0.45, and there is good velocity agreement with CO emission from MBM 151. To the best of our knowledge, this 1.85 km s^-1 component toward HD 157841 is the first one found to have relative line widths that are consistent with pure thermal broadening only. We associate the 1.85 km s^-1 cloud seen in our UHRF spectra with MBM 151 and conclude that HD 157841 must lie beyond ~200 pc, the estimated distance to MBM 151.

High-resolution spectra have been obtained for the regions of the five strongest optical lines of Pt II in the spectrum of the cool HgMn star HR 7775, which is one of the sharpest-lined HgMn stars known. Model lines have been constructed from the isotopic and hyperfine structure laboratory analysis of Engleman. Abundances of the individual isotopes have been determined from spectrum synthesis. The total abundance of Pt is 4.46 dex greater than the adopted solar abundance. The isotopic composition is clearly nonterrestrial, with a pronounced relative enhancement of the heaviest isotope,¹⁹⁸Pt, and deficiencies of isotopes lighter than ¹⁹⁶Pt. The pattern of isotopic composition does not follow the widely assumed fractionation formalism; the lighter isotopes are far more deficient than a single-parameter fractionation pattern would predict.

The solar evolution has been calculated including all the effects of the diffusion of helium and heavy elements. Monochromatic opacities are used to calculate radiative accelerations and Rosseland opacities at each evolution time step, taking into account the local abundance changes of all important (21) chemical elements. The OPAL monochromatic data are used for the opacities and the radiative accelerations. The Opacity Project data are needed to calculate how chemical species and electrons share the momentum absorbed from the radiation flux.

A detailed evaluation of the impact of atomic diffusion on solar models is presented. On some elements thermal diffusion adds approximately 50% to the gravitational settling velocity. While gravitational settling had been included in previous solar models, this is the first time that the impact of radiative accelerations is considered. Radiative accelerations can be up to 40% of gravity below the solar convection zone and thus affect chemical element diffusion significantly, contrary to current belief.

Up to the solar age, the abundances of most metals change by 7.5% if complete ionization is assumed, but by 8.5%-10% if detailed ionization of each species is taken into account. If radiative accelerations are included, intermediate values are obtained. Diffusion leads to a change of up to 8% in the Rosseland opacities, compared to those of the original mixture. Most of this effect can be taken into account by using tables with several values of Z.

If one isolates the effects of radiative accelerations, the abundance changes they cause alter the Rosseland opacity by up to 0.5%; the density is affected by up to 0.2%; the sound speed is affected by at most 0.06%. The inclusion of radiative accelerations leads to a reduction of 3% of neutrino fluxes measured with ³⁷Cl detectors and 1% measured with ⁷¹Ga detectors.

The partial transformation of C and O into N by nuclear reactions in the core causes a ~1% change in the opacities that cannot be modeled by a change in Z alone.

The evolution is allowed to proceed to 10¹⁰ yr in order to determine the impact at the end of the main-sequence life of solar-type stars. It is found that immediately below the convection zone, the radiative acceleration on some iron peak elements is within a few percent of gravity. The abundance anomalies reach 18% for He in the convection zone but are kept within 12% and 15% for most because of g_rad. They would have reached 18% in the absence of g_rad.

Consistent stellar evolution models of F stars (1.1-1.5 M_☉) are calculated with radiative forces, opacities, and diffusion for all elements included in OPAL's opacity tables. The opacities and radiative forces are continuously recomputed during evolution from OPAL's monochromatic data (~1.5 Gbyte) in order to include all effects of abundance changes due to diffusion and nuclear evolution. TOPbase is also used for radiative accelerations. Iron surface overabundances occur in stars more massive than 1.3 M_☉. Local overabundances of iron peak elements increase the Rosseland opacity in a region at the base of the convection zone by a factor of 3-6; this increases the mass of the convective zone by up to a factor of 5. It is important to follow Cr, Mn, and Ni independently of Fe, since they peak at different temperatures within the star. The predicted abundance anomalies are much larger than observed in most F-type stars of open clusters. This suggests that atomic diffusion is not the only process responsible for the Li gap in open clusters. The predicted iron peak element overabundances indicate trends that are compatible with those observed in Fm stars. They however tend to be larger than the observed overabundances, leaving room for some perturbing hydrodynamical process. The present models, devoid of free parameters, are a necessary step in constraining the additional hydrodynamical processes required to better reproduce observed surface abundances. Since the abundances of 28 elements are calculated, one may have 27 constraints on stellar hydrodynamics, once the relative abundances of all species have been determined observationally.

Using spectra obtained from the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) spectrometer flown on the SolarandHeliosphericObservatory (SOHO) spacecraft, we determine the Si/Ne abundance ratio in diffuse, interplume polar coronal hole regions, as well as the ratio relative to quiet-Sun coronal regions. Ne has the second highest first ionization potential (FIP) of solar abundant elements, and Si is a low-FIP element. Thus the Si/Ne ratio is a sensitive indicator of abundance variations due to the FIP effect. We develop new spectroscopic diagnostics for the determination of the Si/Ne abundance ratio. Assuming ionization equilibrium, we find that the Si/Ne abundance ratio in interplume polar coronal hole regions is about a factor of 2 greater than the photospheric value and is close to or the same as in coronal quiet-Sun regions. This result pertains to the electron temperature range 5-8 × 10⁵ K. However, the combined atomic physics, instrumental, and statistical uncertainty in this result is about a factor of 2, and therefore this observed enhancement is consistent with no enhancement in the polar hole abundances. Nevertheless, our results follow the same trend, i.e., a greater than photospheric abundance ratio of low-FIP elements in the corona relative to high-FIP elements, as found from other abundance measurements in the corona that involve different atomic physics and different instruments. Therefore we feel that our results reflect an actual abundance enhancement, despite being within an uncertainty level bar that encompasses photospheric abundances. We also examine the Ne/Mg abundance ratio over a 24.5 hr observation and find no significant abundance variations. (Mg is a low-FIP element.) Thus, no large transient abundance variations appear to occur on timescales shorter than about a day, although this result is based on only one observation. From lines of Mg VII, Mg VIII, Mg IX, and Mg X we find that the electron temperature along the line of sight increases with height above the limb over the polar coronal holes, as has been previously reported. We determine the emission measure distribution as a function of height from Mg VII, Mg VIII, and Mg X lines. We determine average temperatures along the line of sight over the polar holes from Ne VIII/Ne VII, Mg VIII/Mg VII, and Si VIII/Si VII line ratios. We also discuss the temperature properties of the coronal hole and quiet-Sun regions using forbidden lines of Fe X and Fe XI. We comment on the possibility that ionization equilibrium is not valid in polar coronal hole regions, a possible scenario in light of recent observations that show outflows in coronal holes beginning at about the temperature of formation of Ne VIII.

We describe the characteristics of the 350 μm polarimeter Hertz learned from laboratory tests and recent observations at the Caltech Submillimeter Observatory. Hertz contains a pair of 32 element arrays with 18'' pixel spacing and 20'' resolution. The instrument has been improved since initial observations in 1994 and 1995; the detector noise is now below the sky background noise. In excellent weather conditions on Mauna Kea, the noise-equivalent flux density (NEFD) for the measurement of polarized flux is 3-4 Jy Hz^-1/2. The subtraction of correlated sky noise accomplished by the two-array design is crucial for achieving this performance. A method for analysis of our polarization data in the presence of the correlated noise is described. The instrumental polarization of Hertz is less than 0.5% across the detector array. Systematic errors in the measurement of polarization are less than 0.2%. We present a 350 μm polarization map of Sgr B2 with 140 detections at greater than 3 σ significance. For our current database of all 350 μm polarization measurements, the median polarization is 1.1%.

Using an Eulerian perturbative calculation, we show that the distribution of relative pairwise velocities that arises from the gravitational instability of Gaussian density fluctuations has asymmetric (skewed) exponential tails. The negative skewness is induced by the negative mean streaming velocity of pairs (the infall prevails over expansion), while the exponential tails arise because the relative pairwise velocity is a number-weighted, not a volume-weighted, statistic. The derived probability distribution is compared with N-body simulations and is shown to provide a reasonable fit.

We have obtained mid-infrared images of the nucleus of NGC 1068 from the Hale 5 m telescope at Mount Palomar with diffraction-limited resolution and high sensitivity at λ=8.8, 10.3, and 12.5 μm. Deconvolved images show that the infrared emission extends north to south in the inner 2'', consisting of a central peak, a component extending 1'' north of the central source, a component extending 1'' south of the central source, and several smaller structures located 1'' to the northeast. The central peak is extended 04 N-S and unresolved (≤02) E-W. We find that 50%±5% of the flux emerges from the central 04 and that a single unresolved point source can account for only 27%±5% of the total flux. However, if the central peak arises from optically thick emission, we estimate that the emitting region has a projected area ≥2 pc² and thus may contain a compact source such as a parsec-scale torus. We observe a correspondence between the northern extension and northeastern sources appearing on the mid-infrared images and the [O III] clouds A-C and E. We interpret the faint optical counterpart to the mid-infrared southern extension as being due to a partial obscuration by the intervening disk of the host galaxy. The N-S extension of the mid-infrared emission coincides with one wall of the conical narrow-line region and aligns with the N-S orientation of the radio jet close to the nucleus. We interpret the infrared emission as arising from the optically thick dust that lines the walls of the low-density cavity formed by the radio jet and heated by radiation from the central source.

We report measurements of the [C II] 157.74 μm fine-structure line in a sample of seven ultraluminous infrared galaxies (ULIGs) (L_IR > 10¹²L_☉) with the Long Wavelength Spectrometer on the Infrared Space Observatory. The [C II] line is an important coolant in galaxies and arises in interstellar gas exposed to far-ultraviolet photons (hν≥11.26 eV); in ULIGs, this radiation stems from the bursts of star formation and/or from the active galactic nuclei that power the tremendous infrared luminosity. The [C II] 158 μm line is detected in four of the seven ULIGs; the absolute line flux (about a few times 10^-20 W cm^-2) represents some of the faintest extragalactic[C II] emission yet observed. Relative to the far-infrared continuum, the [C II] flux from the observed ULIGs is ~10% of that seen from nearby normal and starburst galaxies. We discuss possible causes for the [C II] deficit, namely (1) self-absorbed or optically thick [C II] emission, (2) saturation of the [C II] emission in photodissociated gas with high gas density n (≫3 × 10³ cm^-3) or with a high ratio of incident UV flux G₀ to n (G₀/n ≳ 10 cm³), or (3) the presence of a soft ultraviolet radiation field caused, for example, by a stellar population deficient in massive main-sequence stars. As nearby examples of colliding galaxies, ULIGs may resemble high-redshift protogalaxies in both morphology and spectral behavior. If true, the suggested [C II] deficit in ULIGs poses limitations on the detection rate of high-z sources and on the usefulness of [C II] as an eventual tracer of protogalaxies.

We present deep Keck telescope spectroscopy of eight galaxies in the luminous X-ray cluster MS 1054-03 at z=0.83. The data are combined with imaging observations from the Hubble Space Telescope (HST). The spectroscopic data are used to measure the internal kinematics of the galaxies, and the HST data are used to measure their structural parameters. Six galaxies have early-type spectra, and two have "E+A" spectra. The galaxies with early-type spectra define a tight fundamental plane (FP) relation. The evolution of the mass-to-light ratio is derived from the FP. The M/L ratio evolves as ΔlogM/L_B ∝ -0.40z (Ω_m=0.3, Ω_Λ=0). The observed evolution of the M/L ratio provides a combined constraint on the formation redshift of the stars, the initial mass function (IMF), and cosmological parameters. For a Salpeter IMF (x=2.35), we find that z_form > 2.8 and Ω_m < 0.86 with 95% confidence. The constraint on the formation redshift is weaker if Ω_Λ > 0: z_form > 1.7 if Ω_m=0.3 and Ω_Λ=0.7. At present, the limiting factor in constraining z_form and Ω from the observed luminosity evolution of early-type galaxies is the poor understanding of the IMF. We find that if Ω_m=1, the IMF must be significantly steeper than the Salpeter IMF (x > 2.6).

The edge-on, nearby spiral galaxy NGC 5907 has long been used as the prototype of a "noninteracting" warped galaxy. We report here the discovery of two interactions with companion dwarf galaxies that substantially change this picture. First, a faint ring structure is discovered around this galaxy that is likely due to the tidal disruption of a companion dwarf spheroidal galaxy. The ring is elliptical in shape with the center of NGC 5907 close to one of the ring's foci. This suggests that the ring material is in orbit around NGC 5907. No gaseous component to the ring has been detected either with deep Hα images or in Very Large Array H I 21 cm line maps. The visible material in the ring has an integrated luminosity ≤10⁸L_☉, and its brightest part has a color R-I~0.9. All of these properties are consistent with the ring being a tidally disrupted dwarf spheroidal galaxy. Second, we find that NGC 5907 has a dwarf companion galaxy, PGC 54419, which is projected to be only 36.9 kpc from the center of NGC 5907, close in radial velocity (ΔV=45 km s^-1) to the giant spiral galaxy. This dwarf is seen at the tip of the H I warp and in the direction of the warp. Hence, NGC 5907 can no longer be considered noninteracting but is obviously interacting with its dwarf companions much as the Milky Way interacts with its dwarf galaxies. These results, coupled with the finding by others that dwarf galaxies tend to be found around giant galaxies, suggest that tidal interaction with companions, even if containing a mere 1% of the mass of the parent galaxy, might be sufficient to excite the warps found in the disks of many large spiral galaxies.

We report the discovery of phase shifts between X-ray pulses at different energies in the newly discovered millisecond X-ray pulsar SAX J1808.4-3658. The results show that low-energy pulses lag high-energy pulses by as much as ~0.2 ms (or ~8% of the pulse period). The measurements were made in two different ways: (1) computing cross-power spectra between different energy bands, and (2) cross-correlating the folded pulse profiles in different energy bands; consistent results were obtained. We speculate that the observed soft lags might be related to the lateral expansion and subsequent cooling of a "hot spot" on the neutron star surface in which the pulsed X-ray emission originates. Also presented is the possibility of producing soft lags via Compton downscattering of hard X-ray photons from the hot spot in the cool surrounding atmosphere. We will discuss possible X-ray production mechanisms for SAX J1808.4-3658 and constraints on the emission environment, based on the observed soft lags, pulse profiles, and energy spectrum.

We report on the implications of the peak in the cosmic star formation rate (SFR) at redshift z ≈ 1.5 for the resulting population of low-mass X-ray binaries (LMXBs) and for that of their descendants, the millisecond radio pulsars (MRPs). Since the evolutionary timescales of LMXBs, their progenitors, and their descendants are thought be significant fractions of the time interval between the SFR peak and the present epoch, there is a lag in the turn-on of the LMXB population, with the peak activity occurring at z~0.5-1. The peak in the MRP population is delayed further, occurring at z≲0.5. We show that the discrepancy between the birthrate of LMXBs and the birthrate of MRPs, found under the assumption of a steady state SFR, can be resolved for the population as a whole when the effects of a time-variable SFR are included. A discrepancy may persist for LMXBs with short orbital periods, although a detailed population synthesis will be required to confirm this. Furthermore, since the integrated X-ray luminosity distribution of normal galaxies is dominated by X-ray binaries, it should show strong luminosity evolution with redshift. In addition to an enhancement near the peak (z ≈ 1.5) of the SFR due to the prompt turn-on of the relatively short-lived massive X-ray binaries and young supernova remnants, we predict a second enhancement by a factor of ~10 at a redshift between ~0.5 and ~1 due to the delayed turn-on of the LMXB population. Deep X-ray observations of galaxies out to z ≈ 1 by the Advanced X-Ray Astrophysics Facility will be able to observe this enhancement and, by determining its shape as a function of redshift, will provide an important new method for constraining evolutionary models of X-ray binaries.

We discovered two simultaneous kHz quasi-periodic oscillations (QPOs) in the bright low-mass X-ray binary and Z source GX 5-1 with the Rossi X-ray Timing Explorer. In the X-ray color-color and hardness-intensity diagram a clear Z track is traced out, which shifted between observations. The frequencies of the two kHz QPOs increased from ~215 and ~500 Hz on the left part of the horizontal branch to ~700 and ~ 890 Hz, respectively, on the upper part of the normal branch. With increasing frequency the FWHM and rms amplitude (8.6-60 keV) of the higher frequency kHz QPO decreased from 300 to 30 Hz, and from 6.6% to 2.4%, respectively. The FWHM and amplitude of the lower frequency kHz QPO (50-100 Hz and 3%-4% rms) did not correlate with the position of the source on the Z track. The kHz QPO separation was consistent with being constant at 298±11 Hz. Simultaneously with the kHz QPOs horizontal branch oscillations were detected with frequencies between 18 and 56 Hz.

We report on a near-infrared, long-baseline interferometric search for luminous companions to the star 51 Pegasi conducted with the Palomar Testbed Interferometer. Our data is completely consistent with a single-star hypothesis. We find no evidence to suggest a luminous companion to 51 Pegasi, and we can exclude a companion brighter than a ΔK of 4.27 at the 99% confidence level for the 4.2 day orbital period indicated by spectroscopic measurements. This ΔK corresponds to an upper limit in the companion M_K of 7.30, which in turn implies a main-sequence companion mass of less than 0.22 M_☉.

We present long-slit 7.5-13.5 μm spectra of WL 16, a Herbig Ae star in the ρ Ophiuchi dark cloud that is surrounded by a ~1000 AU diameter infrared-emitting nebula. Mid-IR emission features are detected in every region of the nebula from center to edge, and the underlying continuum color temperature is approximately 10 times hotter than would be expected from large grains in thermal equilibrium. The 7.7 and 8.6 μm features decrease in intensity more rapidly with distance from the central star than the 11.3 and 12.7 μm features, suggesting that PAH ionization effects proposed to explain similar spectral variations in other ISM sources may be important in WL 16 as well.

We have reanalyzed the Goddard High Resolution Spectrograph data set presented by Snow et al., which contains the interstellar intersystem C II] λ2325 line through the translucent cloud toward HD 24534 (X Persei). In contrast to the results of Snow et al., we clearly detect the C II] feature at the 3 σ confidence level and measure a C⁺ column density of 2.7±0.8×10¹⁷ cm^-2. Accounting for the C I column density along the line of sight, we find 10⁶ C/H=106±38 in the interstellar gas toward this star. This gas-phase carbon-to-hydrogen ratio suggests that slightly more carbon depletion may be occurring in translucent as compared to diffuse clouds. The average diffuse-cloud C/H, however, is within the 1 σ uncertainty of the measurement toward HD 24534. We therefore cannot rule out the possibility that the two cloud types have comparable gas-phase C/H and therefore comparable depletions of carbon.

The power spectrum of more than 630 days of full-disk solar velocity data, provided by the GOLF spectrophotometer aboard the Solar and Heliospheric Observatory, has revealed the presence of modelike structure well beyond the acoustic cutoff frequency for the solar atmosphere (ν_ac ~5.4 mHz). Similar data produced by full-disk instruments deployed in Earth-based networks (BiSON and IRIS) had not shown any peak structure above ν_ac: this is probably due to the higher levels of noise that are inherent in Earth-based experiments. We show that the observed peak structure (ν_ac≤ν≤7.5 mHz) can be explained by a simple two-wave interference model if the high-frequency waves are partially reflected at the back side of the Sun.