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Research Interests: Physics, Astrochemistry, Dust (Astronomy & Astrophysics), Herschel Space Observatory, Astrophysics, and 11 moreMolecular Clouds, Iron, Spectrum, Infrared, Far Infrared, Radiative Transfer, Acoustic Radiative Transfer Model, Chemical Composition, Pre main Sequence Stars, Photosphere, and Principal Investigator
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For several hours on 2008 April 19 the T Tauri spectroscopic binary DQ Tau was observed to brighten, reaching a maximum detected flux of 468 mJy and likely making it (briefly) the brightest object at 3 mm in the Taurus star-forming... more
For several hours on 2008 April 19 the T Tauri spectroscopic binary DQ Tau was observed to brighten, reaching a maximum detected flux of 468 mJy and likely making it (briefly) the brightest object at 3 mm in the Taurus star-forming region. We present the light curve of a rarely before observed millimeter flare originating in the region around a pre-main-sequence star, and the first from a classical T Tauri star. We discuss the properties and nature of the flaring behavior in the context of pulsed accretion flows (the current picture based largely on studies of this object's optically variable spectrum), as well as magnetospheric re-connection models (a separate theory that predicts millimeter flares for close binaries of high orbital eccentricity). We believe that the flare mechanism is linked to the binary orbit, and therefore periodic. DQ Tau makes a strong case for multi-wavelength follow-up studies, performed in parallel, of future flares to help determine whether magnetosph...
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We present IRAC (3.6, 4.5, 5.8, and 8.0 micron) observations of the Chamaeleon II molecular cloud. The observed area covers about 1 square degree defined by A_V >2. Analysis of the data in the 2005 c2d catalogs reveals a small number... more
We present IRAC (3.6, 4.5, 5.8, and 8.0 micron) observations of the Chamaeleon II molecular cloud. The observed area covers about 1 square degree defined by A_V >2. Analysis of the data in the 2005 c2d catalogs reveals a small number of sources (40) with properties similar to those of young stellaror substellar objects (YSOs). The surface density of these YSO candidates is low, and contamination by background galaxies appears to be substantial, especially for sources classified as Class I or flat SED. We discuss this problem in some detail and conclude that very few of the candidate YSOs in early evolutionary stages are actually in the Cha II cloud. Using a refined set of criteria, we define a smaller, but more reliable, set of 24 YSO candidates.
We present the first images of the 691.473 GHz CO J=6–5 line in a protoplanetary disk, obtained along with the 690 GHz dust continuum, toward the classical T Tauri star TW Hya using the Submillimeter Array. Imaging in the CO J=6–5 line... more
We present the first images of the 691.473 GHz CO J=6–5 line in a protoplanetary disk, obtained along with the 690 GHz dust continuum, toward the classical T Tauri star TW Hya using the Submillimeter Array. Imaging in the CO J=6–5 line reveals a rotating disk, consistent with previous observations of CO J=3–2 and 2–1 lines. Using an irradiated accretion disk model and 2D Monte Carlo radiative transfer, we find that additional surface heating is needed to fit simultaneously the absolute and relative intensities of the CO J=6–5, 3–2 and 2–1 lines. In particular, the vertical gas temperature gradient in the disk must be steeper than that of the dust, mostly likely because the CO emission lines probe nearer to the surface of the disk. We have used an idealized X-ray heating model to fit the line profiles of CO J=2–1 and 3–2 with χ 2 analysis, and the prediction of this model yields CO J=6–5 emission consistent with the observations.
Abstract. Motivated by Yung and Miller’s [1997] sugges-tion that N2O is isotopically fractionated during UV pho-tolysis in the stratosphere, we have studied the photoly-sis rates of the 14N15N16O and 15N14N16O structural iso-topomers. In... more
Abstract. Motivated by Yung and Miller’s [1997] sugges-tion that N2O is isotopically fractionated during UV pho-tolysis in the stratosphere, we have studied the photoly-sis rates of the 14N15N16O and 15N14N16O structural iso-topomers. In this study, we follow the concentrations of these compounds with FTIR spectroscopy during photol-ysis at 213 nm. When fitted to a Rayleigh fractionation model, the observations yield single-stage enrichment fac-tors of (14N15N16O / 14N14N16O) =-73 ± 5 per mil and (15N14N16O / 14N14N16O) =-41 ± 10 per mil. As predicted by Yung and Miller [1997], the photolysis rate of 15N14N16O is faster than 14N15N16O at this wavelength. The magnitude of the observed fractionation, however, is significantly larger than predicted.
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Our study began with the goal of developing new methods to test the radically new understanding of solar system formation that has recently emerged, and to identify innovative instrumentation targeted to this purpose. In particular, we... more
Our study began with the goal of developing new methods to test the radically new understanding of solar system formation that has recently emerged, and to identify innovative instrumentation targeted to this purpose. In particular, we were seeking to test predictions of dynamical models such as the Nice model, and to do so through interdisciplinary collaboration between the planetary dynamics communities that have formulated (and largely dominated discussion of) these new ideas, and the meteoritics and cosmochemistry communities who will be most involved in any in-situ mission to an outer solar system body. Our study was principally focused on coming up with explicit tests of the predictions of these new dynamical models of solar system evolution.
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Motivated by Yung and Miller's [1997] suggestion that N2O is isotopically fractionated during UV photolysis in the stratosphere, we have studied the photolysis rates of the ^(14)N^(15)N^(16)O and ^(15)N^(14)N^(16)O structural... more
Motivated by Yung and Miller's [1997] suggestion that N2O is isotopically fractionated during UV photolysis in the stratosphere, we have studied the photolysis rates of the ^(14)N^(15)N^(16)O and ^(15)N^(14)N^(16)O structural isotopomers. In this study, we follow the concentrations of these compounds with FTIR spectroscopy during photolysis at 213 nm. When fitted to a Rayleigh fractionation model, the observations yield single‐stage enrichment factors of ϵ(^(14)N^(15)N^(16)O / ^(14)N^(14)N^(16)O) = −73 ± 5 per mil and ϵ(^(15)N^(14)N^(16)O / ^(14)N^(14)N^(16)O) = −41 ± 10 per mil. As predicted by Yung and Miller [1997], the photolysis rate of ^(15)N^(14)N^(16)O is faster than ^(14)N^(15)N^(16)O at this wavelength. The magnitude of the observed fractionation, however, is significantly larger than predicted.
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Motivated by Yung and Miller's [199• suggestion that N20 is isotopically fractionated during UV photolysis in the stratosphere, we have studied the photolysis rates of the •4N•SN•60 and •SN•4N•60 structural isotopomers. In this study,... more
Motivated by Yung and Miller's [199• suggestion that N20 is isotopically fractionated during UV photolysis in the stratosphere, we have studied the photolysis rates of the •4N•SN•60 and •SN•4N•60 structural isotopomers. In this study, we follow the concentrations of these compounds with FTIR spectroscopy during photolysis at 213 nm. When fitted to a Rayleigh fractionation model, the observations yield single-stage enrichment factors of e(•4N•SN•O / •4N•4N•O) = -73 q5 per mil and e(•SN•4N•O / •4N•4N•O) -41 q10 per mil. As predicted by Yung and Miller [199•, the photolysis rate of •SN•4N•60 is faster than •4N•SN•O at this wavelength. The magnitude of the observed fractionation, however, is significantly larger than predicted.
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The icy mantles of interstellar dust grains are the birthplaces of the primordial prebiotic molecular inventory that may eventually seed nascent solar systems and the planets and planetesimals that form therein. Here, we present a study... more
The icy mantles of interstellar dust grains are the birthplaces of the primordial prebiotic molecular inventory that may eventually seed nascent solar systems and the planets and planetesimals that form therein. Here, we present a study of two of the most abundant species in these ices after water: carbon dioxide (CO2) and methanol (CH3OH), using TeraHertz (THz) time-domain spectroscopy and mid-infrared spectroscopy. We study pure and mixed-ices of these species, and demonstrate the power of the THz region of the spectrum to elucidate the long-range structure (i.e. crystalline versus amorphous) of the ice, the degree of segregation of these species within the ice, and the thermal history of the species within the ice. Finally, we comment on the utility of the THz transitions arising from these ices for use in astronomical observations of interstellar ices.
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Using rotational spectroscopy, we report characterization of two iso-propanol–water dimers. We further characterize the dynamics of one isomer.
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Previous theoretical work on the ethanol–methanol dimer has been inconclusive in predicting the preferred hydrogen bond donor/acceptor configuration.
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We report the first rotational spectrum of the ground state of the isolated ethanol–water dimer using chirped-pulse Fourier transform microwave spectroscopy between 8–18 GHz.
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We present Submillimeter Array observations of several deuterated species in the disk around the classical T Tauri star TW Hydrae at arcsecond scales, including detections of the DCN J=3-2 and DCO+ J=3-2 lines and upper limits to the HDO... more
We present Submillimeter Array observations of several deuterated species in the disk around the classical T Tauri star TW Hydrae at arcsecond scales, including detections of the DCN J=3-2 and DCO+ J=3-2 lines and upper limits to the HDO 31,2-22,1, ortho-H2D+ 11,0-11,1, and para-D2H+ 11,0-10,1 transitions. We also present observations of the HCN J=3-2, HCO+ J=3-2, and H13CO+ J=4-3 lines for comparison with their deuterated isotopologues. We constrain the radial and vertical distributions of various species in the disk by fitting the data using a model where the molecular emission from an irradiated accretion disk is sampled with a two-dimensional Monte Carlo radiative transfer code. We find that the distribution of DCO+ differs markedly from that of HCO+. The D/H ratios inferred change by at least 1 order of magnitude (0.01-0.1) for radii =70 AU, and there is a rapid falloff of the abundance of DCO+ at radii larger than 90 AU. Using a simple analytical chemical model, we constrain t...
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Recent results indicate that the grain size and crystallinity inferred from observations of silicate features may be correlated with the spectral type of the central star and/or disk geometry. In this paper, we show that grain size, as... more
Recent results indicate that the grain size and crystallinity inferred from observations of silicate features may be correlated with the spectral type of the central star and/or disk geometry. In this paper, we show that grain size, as probed by the 10 m silicate feature peak-to-continuum ...
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Aims: We aim to study the composition and energetics of the circumstellar material of DK Cha, an intermediate-mass star in transition from an embedded configuration to a star plus disk stage, during this pivotal stage of its evolution.... more
Aims: We aim to study the composition and energetics of the circumstellar material of DK Cha, an intermediate-mass star in transition from an embedded configuration to a star plus disk stage, during this pivotal stage of its evolution. Methods: Using the range scan mode of PACS on the Herschel Space Observatory, we obtained a spectrum of DK Cha from 55 to 210 μm as part of the DIGIT key program. Results: Almost 50 molecular and atomic lines were detected, many more than the 7 lines detected in ISO-LWS. Nearly the entire ladder of CO from J = 14-13 to 38-37 (Eu/k = 4080 K), water from levels as excited as JK_{-1K+1} = 707 (Eu/k = 843 K), and OH lines up to Eu/k = 290 K were detected. Conclusions: The continuum emission in our PACS SED scan matches the flux expected by a model consisting of a star, a surrounding disk of 0.03 M_⊙, and an envelope of a similar mass, supporting the suggestion that the object is emerging from its main accretion stage. Molecular, atomic, and ionic emission...
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We present 2D terahertz-terahertz-Raman (2D TTR) spectroscopy, the first technique, to our knowledge, to interrogate a liquid with multiple pulses of terahertz (THz) light. This hybrid approach isolates nonlinear signatures in isotropic... more
We present 2D terahertz-terahertz-Raman (2D TTR) spectroscopy, the first technique, to our knowledge, to interrogate a liquid with multiple pulses of terahertz (THz) light. This hybrid approach isolates nonlinear signatures in isotropic media, and is sensitive to the coupling and anharmonicity of thermally activated THz modes that play a central role in liquid-phase chemistry. Specifically, by varying the timing between two intense THz pulses, we control the orientational alignment of molecules in a liquid, and nonlinearly excite vibrational coherences. A comparison of experimental and simulated 2D TTR spectra of bromoform (CHBr3), carbon tetrachloride (CCl4), and dibromodichloromethane (CBr2Cl2) shows previously unobserved off-diagonal anharmonic coupling between thermally populated vibrational modes.
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High resolution NIR spectroscopy offers an excellent complement to the expanding dataset of transit and secondary eclipse observations of exo-planets with Spitzer that have provided the bulk of our understanding of the atmospheres and... more
High resolution NIR spectroscopy offers an excellent complement to the expanding dataset of transit and secondary eclipse observations of exo-planets with Spitzer that have provided the bulk of our understanding of the atmospheres and internal structure of these objects. High-resolution data can quantify the vertical temperature structure by isolating specific spectral lines formed at various depths. The presence of an
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Recent advances in the observational characterization of young stellar objects (YSOs) with millimeter-wave aperture synthesis arrays and (sub)millimeter single dish telescopes are reviewed. Studies of circumstellar material with ∼arc... more
Recent advances in the observational characterization of young stellar objects (YSOs) with millimeter-wave aperture synthesis arrays and (sub)millimeter single dish telescopes are reviewed. Studies of circumstellar material with ∼arc second resolution, which have only become possible at these frequencies within the past few years and which can potentially probe all stages of the star formation process, are emphasized. Molecules that are sensitive to different routes of formation and modification are outlined, including comments about their utility in distinguishing between a variety of environments and histories in star-forming cloud cores.
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High-degree time-multiplexed multifocal multiphoton microscopy was expected to provide a facile path to scanningless optical-sectioning and the fast imaging of dynamic three-dimensional biological systems. However, physical constraints on... more
High-degree time-multiplexed multifocal multiphoton microscopy was expected to provide a facile path to scanningless optical-sectioning and the fast imaging of dynamic three-dimensional biological systems. However, physical constraints on typical time multiplexing devices, arising from diffraction in the free-space propagation of light waves, lead to significant manufacturing difficulties and have prevented the experimental realization of high-degree time multiplexing. To resolve this issue, we have developed a novel method using optical fiber bundles of various lengths to confine the diffraction of propagating light waves and to create a time multiplexing effect. Through this method, we experimentally demonstrate the highest degree of time multiplexing ever achieved in multifocal multiphoton microscopy (~50 times larger than conventional approaches), and hence the potential of using simply-manufactured devices for scanningless optical sectioning of biological systems.
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Broadband spectroscopy is an invaluable tool for measuring multiple gas-phase species simultaneously. In this work we review basic techniques, implementations, and current applications for broadband spectroscopy. We discuss components of... more
Broadband spectroscopy is an invaluable tool for measuring multiple gas-phase species simultaneously. In this work we review basic techniques, implementations, and current applications for broadband spectroscopy. We discuss components of broad-band spectroscopy including light sources, absorption cells, and detection methods and then discuss specific combinations of these components in commonly-used techniques. We finish this review by discussing potential future advances in techniques and applications of broad-band spectroscopy.
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Observations of submillimeter lines of CO, HCO+, HCN and their isotopes from circumstellar disks around low-mass pre-main sequence stars can be used to set constraints on the temperature and density distributions in these disks. The lines... more
Observations of submillimeter lines of CO, HCO+, HCN and their isotopes from circumstellar disks around low-mass pre-main sequence stars can be used to set constraints on the temperature and density distributions in these disks. The lines considered here originate from levels with higher excitation temperatures and critical densities than studied before (CO 6–5, HCO+ and HCN 4–3), and are combined with interferometer data on lower excitation lines. We discuss the results for two disks, i.e., those around LkCa 15 and TW Hya. We find that the TW Hya disk has a warm surface layer and agrees well with a flaring disk geometry, while the LkCa 15 disk is cooler and can be described by either dust-settling in a flared disk or a flatter disk overall. The densities are well described by disk models in the literature.
Circumstellar disks have now been found around many young stars and a significant fraction of 1-5 Myr old T Tauri stars have been shown to possess accretion disks with masses ( ~ 10-3-10-1 MSun) and sizes ( ~ 100AU) comparable to those... more
Circumstellar disks have now been found around many young stars and a significant fraction of 1-5 Myr old T Tauri stars have been shown to possess accretion disks with masses ( ~ 10-3-10-1 MSun) and sizes ( ~ 100AU) comparable to those inferred for the primitive solar nebula (Beckwith & Sargent 1996). Thus, models of the chemical composition at each radius of these disks will provide valuable information (i.e. density, thermal history and composition) about the initial condition of the solar nebula. In recent years, molecular emission studies of accretion disks have been shown to facilitate the quantification of gas-to-dust ratios and the timescales over which they are dissipated (important for the process of planet formation) as well as the description of the distribution of volatile species in its outer regions (necessary in the derivation of the evolution of comets and Kuiper Belt Objects). Extensive molecular modeling of the chemistry of accretion disks is clearly codependent upon the observational study of the disks through molecular line emission. In this study, the molecular emission from four accretion disks around both low-mass TTauri stars (LkCa 15 and MWC 480) and moderate-mass Herbig Ae stars (HD 163296 and MWC 480) has been observed using the Owen's Valley Radio Observatory (OVRO) and the Caltech Submilimeter Observatory (CSO). These studies include emission from several predicted gas-phase C-,O-,S-,N-bearing molecules. The spectra obtained have been compared to those predicted through models produced via the couping of radiative transfer codes developed by Hogerheijde & van der Tak (2000) and models of circumstellar disk molecular abundances (Willacy & Langer 2000) in order to obtain optical depth and excitation information. In addition, the OVRO maps were also compared to maps produced by the model discussed above. This work was supported by the NASA Graduate Student Researchers Program, NGT5-50231, and the National Science Foundation, AST 9981546.
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We propose to search for methyl acetate, CH3COOCH3, in high mass hot cores. Methyl acetate is possibly synthesized through multiple reaction pathways from molecules previously detected in hot cores, most notably from acetic acid and... more
We propose to search for methyl acetate, CH3COOCH3, in high mass hot cores. Methyl acetate is possibly synthesized through multiple reaction pathways from molecules previously detected in hot cores, most notably from acetic acid and methanol via esterification. Esterification, beyond the formation of methyl formate, has not yet been observed in the ISM. The project is already underway in nothern