Publications (14)12.05 Total impact
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Article: Resolving the Sub-AU-Scale Gas and Dust Distribution in FU Orionis Sources
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ABSTRACT: We present Keck Interferometer observations of the three prototypical FU Orionis stars, FU Ori, V1057 Cyg, and V1515 Cyg. With a spatial resolution of a few milli-arcseconds and a spectral resolution of 2000, our near-infrared observations spatially resolve gas and dust emission extending from stellocentric radii of ~0.05 AU to several AU. We fit these data with accretion disk models where each stellocentric radius of the disk is represented by a supergiant-type stellar emission spectrum at the disk temperature. A disk model is consistent with the data for FU Ori, although we require some local asymmetry in the disk. For V1057 Cyg the disk model does not fit our data well, especially compared to the fit quality achieved for FU Ori. We speculate that a disk wind may be contributing substantially to the observed near-IR emission in this source. The data for V1515 Cyg are noisier than the data obtained for the other two objects, and do not strongly constrain the validity of an accretion disk model.06/2011; -
Article: Probing Local Density Inhomogeneities in the Circumstellar Disk of a Be Star Using the New Spectro-astrometry Mode at the Keck Interferometer
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ABSTRACT: We report on the successful science verification phase of a new observing mode at the Keck Interferometer, which provides a line-spread function width and sampling of 150 km s–1 at the K'-band, at a current limiting magnitude of K' ~ 7 mag with a spatial resolution of λ / 2 B 2.7 mas and a measured differential phase stability of unprecedented precision (3 mrad at K = 5 mag, which represents 3 μ as on the sky or a centroiding precision of 10–3). The scientific potential of this mode is demonstrated by the presented observations of the circumstellar disk of the evolved Be-star 48 Lib. In addition to indirect methods such as multi-wavelength spectroscopy and polarimetry, the spectro-interferometric astrometry described here provides a new tool to directly constrain the radial density structure in the disk. For the first time, we resolve several Pfund emission lines, in addition to Br γ, in a single interferometric spectrum, with adequate spatial and spectral resolution and precision to analyze the radial disk structure in 48 Lib. The data suggest that the continuum and Pf-emission originates in significantly more compact regions, inside the Br γ-emission zone. Thus, spectro-interferometric astrometry opens the opportunity to directly connect the different observed line profiles of Br γ and Pfund in the total and correlated flux to different disk radii. The gravitational potential of a rotationally flattened Be star is expected to induce a one-armed density perturbation in the circumstellar disk. Such a slowly rotating disk oscillation has been used to explain the well-known periodic V/R spectral profile variability in these stars, as well as the observed V/R cycle phase shifts between different disk emission lines. The differential line properties and linear constraints set by our data are consistent with theoretical models and lend direct support to the existence of a radius-dependent disk density perturbation. The data also show decreasing gas rotation velocities at increasing stellocentric radii as expected for Keplerian disk rotation, assumed by those models.The Astrophysical Journal 08/2010; 721(1):802. · 6.02 Impact Factor -
Article: Spatially and Spectrally Resolved Hydrogen Gas within 0.1 AU of T Tauri and Herbig Ae/Be Stars
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ABSTRACT: We present near-infrared observations of T Tauri and Herbig Ae/Be stars with a spatial resolution of a few milli-arcseconds and a spectral resolution of ~2000. Our observations spatially resolve gas and dust in the inner regions of protoplanetary disks, and spectrally resolve broad-linewidth emission from the Brackett gamma transition of hydrogen gas. We use the technique of spectro-astrometry to determine centroids of different velocity components of this gaseous emission at a precision orders of magnitude better than the angular resolution. In all sources, we find the gaseous emission to be more compact than or distributed on similar spatial scales to the dust emission. We attempt to fit the data with models including both dust and Brackett gamma-emitting gas, and we consider both disk and infall/outflow morphologies for the gaseous matter. In most cases where we can distinguish between these two models, the data show a preference for infall/outflow models. In all cases, our data appear consistent with the presence of some gas at stellocentric radii of ~0.01 AU. Our findings support the hypothesis that Brackett gamma emission generally traces magnetospherically driven accretion and/or outflows in young star/disk systems. Comment: 48 pages, including 17 figures. Accepted for publication by ApJ06/2010; -
Article: First L-band Interferometric Observations of a Young Stellar Object: Probing the Circumstellar Environment of MWC 419
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ABSTRACT: We present spatially-resolved K- and L-band spectra (at spectral resolution R = 230 and R = 60, respectively) of MWC 419, a Herbig Ae/Be star. The data were obtained simultaneously with a new configuration of the 85-m baseline Keck Interferometer. Our observations are sensitive to the radial distribution of temperature in the inner region of the disk of MWC 419. We fit the visibility data with both simple geometric and more physical disk models. The geometric models (uniform disk and Gaussian) show that the apparent size increases linearly with wavelength in the 2-4 microns wavelength region, suggesting that the disk is extended with a temperature gradient. A model having a power-law temperature gradient with radius simultaneously fits our interferometric measurements and the spectral energy distribution data from the literature. The slope of the power-law is close to that expected from an optically thick disk. Our spectrally dispersed interferometric measurements include the Br gamma emission line. The measured disk size at and around Br gamma suggests that emitting hydrogen gas is located inside (or within the inner regions) of the dust disk. Comment: Accepted for publication in ApJ07/2009; -
Article: Keck Interferometer Observations of FU Orionis Objects
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ABSTRACT: We present new K-band long-baseline interferometer observations of three young stellar objects of the FU Orionis class, namely, V1057 Cyg, V1515 Cyg, and Z CMa-SE, obtained at the Keck Interferometer during its commissioning science period. The interferometer clearly resolves the source of near-infrared emission in all three objects. Using simple geometric models, we derive size scales (0.5-4.5 AU) for this emission. All three objects appear significantly more resolved than expected from simple models of accretion disks tuned to fit the broadband optical and infrared spectrophotometry. We explore variations in the key parameters that are able to lower the predicted visibility amplitudes to the measured levels and conclude that accretion disks alone do not reproduce the spectral energy distributions and K-band visibilities simultaneously. We conclude that either disk models are inadequate to describe the near-infrared emission or additional source components are needed. We hypothesize that large-scale emission (tens of AU) in the interferometer field of view is responsible for the surprisingly low visibilities. This emission may arise in scattering by large envelopes believed to surround these objects.The Astrophysical Journal 12/2008; 641(1):547. · 6.02 Impact Factor -
Article: Near-Infrared Interferometric, Spectroscopic, and Photometric Monitoring of T Tauri Inner Disks
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ABSTRACT: We present high angular resolution observations with the Keck Interferometer, high dispersion spectroscopic observations with Keck/NIRSPEC, and near-IR photometric observations from PAIRITEL of a sample of 11 solar-type T Tauri stars in 9 systems. We use these observations to probe the circumstellar material within 1 AU of these young stars, measuring the circumstellar-to-stellar flux ratios and angular size scales of the 2.2 micron emission. Our sample spans a range of stellar luminosities and mass accretion rates, allowing investigation of potential correlations between inner disk properties and stellar or accretion properties. We suggest that the mechanism by which the dusty inner disk is truncated may depend on the accretion rate of the source; in objects with low accretion rates, the stellar magnetospheres may truncate the disks, while sublimation may truncate dusty disks around sources with higher accretion rates. We have also included in our sample objects that are known to be highly variable (based on previous photometric and spectroscopic observations), and for several sources, we obtained multiple epochs of spectroscopic and interferometric data, supplemented by near-IR photometric monitoring, to search for inner disk variability. While time-variable veilings and accretion rates are observed in some sources, no strong evidence for inner disk pulsation is found. Comment: 33 pages, 6 figures. Accepted for publication in ApJ07/2007; -
Article: Spatially Resolving the Inner Disk of TW Hya
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ABSTRACT: We present Keck Interferometer observations of TW Hya that spatially resolve its emission at 2 micron wavelength. Analyzing these data together with existing K-band veiling and near-infrared photometric measurements, we conclude that the inner disk consists of optically thin, sub-micron-sized dust extending from ~4 AU to within 0.06 AU of the central star. The inner disk edge may be magnetospherically truncated. Even if we account for the presence of gas in the inner disk, these small dust grains have survival times against radiation blow-out that are orders of magnitude shorter than the age of the system, suggesting continual replenishment through collisions of larger bodies. Comment: 11 pages, including 2 figures. Accepted by ApJL01/2006; -
Article: Constraining the Evolutionary Stage of Class I Protostars: Multi-wavelength Observations and Modeling
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ABSTRACT: (abridged) We present new Keck images at 0.9 micron and OVRO 1.3 mm continuum images of five Class I protostars in the Taurus star forming region. We analyze these data in conjunction with broadband spectral energy distributions and 8-13 micron spectra from the literature using a Monte Carlo radiative transfer code. By fitting models for the circumstellar dust distributions simultaneously to the scattered light images, millimeter continuum data, and the SEDs, we attempt to distinguish between flared disks, infalling envelopes with outflow cavities, and combinations of disks and envelopes. For each of these circumstellar density distributions, we generate grids of models for varying geometries, dust masses, and accretion rates, and determine the best fits by minimizing the residuals between model and data. Models incorporating both massive envelopes and massive embedded disks generally fit the imaging+SED data best. Implied envelope infall rates for these models are consistent with infall rates derived by previous investigators, although they are approximately an order of magnitude larger than inner disk accretion rates inferred from recent spectroscopic measurements. In addition, the disk masses inferred from our models are close to or larger than the limit for gravitationally stable disks, indicating that Class I disks may undergo periodic episodes of enhanced accretion, perhaps as a result of gravitational instabilities. An important caveat to these results is that in some cases, no single model can fit all of the imaging and SED data well, suggesting that further refinements to models of the circumstellar dust distributions around Class I sources are necessary. Comment: 58 pages, including 18 postscript figures. Accepted for publication in ApJ (Dec 10 issue)08/2005; -
Article: Observations of T Tauri Disks at Sub-AU Radii: Implications for Magnetospheric Accretion and Planet Formation
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ABSTRACT: We determine inner disk sizes and temperatures for four solar-type (1-2 M$_{\odot}$) classical T Tauri stars (AS 207A, V2508 Oph, AS 205A, and PX Vul) using 2.2 $\mu$m observations from the Keck Interferometer. Nearly contemporaneous near-IR adaptive optics imaging photometry, optical photometry, and high-dispersion optical spectroscopy are used to distinguish contributions from the inner disks and central stars in the interferometric observations. In addition, the spectroscopic and photometric data provide estimates of stellar properties, mass accretion rates, and disk co-rotation radii. We model our interferometric and photometric data in the context of geometrically flat accretion disk models with inner holes, and flared disks with puffed-up inner walls. Models incorporating puffed-up inner disk walls generally provide better fits to the data, similar to previous results for higher-mass Herbig Ae stars. Our measured inner disk sizes are larger than disk truncation radii predicted by magnetospheric accretion models, with larger discrepancies for sources with higher mass accretion rates. We suggest that our measured sizes correspond to dust sublimation radii, and that optically-thin gaseous material may extend further inward to the magnetospheric truncation radii. Finally, our inner disk measurements constrain the location of terrestrial planet formation as well as potential mechanisms for halting giant planet migration. Comment: Accepted for publication in ApJ (May 1, 2005 issue)01/2005; -
Article: Resolved Inner Disks around Herbig Ae/Be Stars
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ABSTRACT: We have observed 14 Herbig Ae/Be sources with the long-baseline near-IR Palomar Testbed Interferometer, All except two sources are resolved at 2.2 um, with angular sizes generally <5 mas. We determine the size scales and orientations of the 2.2 um emission using various models: uniform disks, Gaussians, uniform rings, flat accretion disks with inner holes, and flared disks with puffed-up inner rims; 7 objects display significantly inclined morphologies, generally compatible with the outer disk geometries inferred from millimeter interferometric observations, implying that HAEBE disks are not significantly warped. Using the derived inner disk sizes and inclinations, we compute the spectral energy distributions for two simple physical disk models, and compare these with observed SEDs compiled from the literature and new near-IR photometry. While geometrically flat accretion disk models are consistent with the data for the earliest spectral types in our sample (MWC 297, V1685 Cyg, and MWC 1080), the later-type sources are explained better through models incorporating puffed-up inner disk walls; this may indicate different accretion mechanisms for early and late-type Herbig Ae/Be stars. Comment: 54 pages, including 16 figures. Accepted for publication in the Astrophysical Journal (October 2004 issue)06/2004; -
Article: Young Stellar Objects with the Keck Interferometer
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ABSTRACT: Here we report first results of a survey of young stellar objects (YSO) with the newly-commissioned Keck Interferometer (Keck-I). The Keck-I has an order-of-magnitude greater sensitivity than other near-infrared interferometers allowing characteristic size measurements of a wide variety of YSO systems. The inner accretion disks of Classical T Tauris, Herbig Ae/Be stars, and FU Orionis objects have been observed at astronomical K-band (2.2 microns) and nearly all targets were resolved. In this poster, we present a status report and preliminary analysis of our initial dataset. This work has been funded by NASA. The interferometer observations were made possible through the efforts of the NASA-Jet Propulsion Laboratory, the W. M. Keck Observatory, and the Michelson Science Center (Caltech).11/2003; 35:1214. -
Article: Near-Infrared Interferometric Measurements of Herbig Ae/Be Stars
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ABSTRACT: We have observed the Herbig Ae/Be sources AB Aur, VV Ser, V1685 Cyg (BD+40 4124), AS 442, and MWC 1080 with the Palomar Testbed Interferometer, obtaining the longest baseline near-IR interferometric observations of this class of objects. All of the sources are resolved at 2.2 microns with angular size scales generally <5 mas, consistent with the only previous near-IR interferometric measurements of Herbig Ae/Be stars by Millan-Gabet and collaborators. We determine the angular size scales and orientations predicted by uniform disk, Gaussian, ring, and accretion disk models. Although it is difficult to distinguish different radial distributions, we are able to place firm constraints on the inclinations of these models, and our measurements are the first that show evidence for significantly inclined morphologies. In addition, the derived angular sizes for the early type Herbig Be stars in our sample, V1685 Cyg and MWC 1080, agree reasonably well with those predicted by the face-on accretion disk models used by Hillenbrand and collaborators to explain observed spectral energy distributions. In contrast, our data for the later-type sources AB Aur, VV Ser, and AS 442 are somewhat inconsistent with these models, and may be explained better through the puffed-up inner disk models of Dullemond and collaborators. Comment: 40 pages, 11 figures. Accepted for publication in ApJ01/2003; -
Article: First Keck Interferometer measurements in self-phase referencing mode: spatially resolving circum-stellar line emission of 48 Lib
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ABSTRACT: Recently, the Keck interferometer was upgraded to do self-phase-referencing (SPR) assisted K-band spectroscopy at R ~ 2000. This means, combining a spectral resolution of 150 km/s with an angular resolution of 2.7 mas, while maintaining high sensitiviy. This SPR mode operates two fringe trackers in parallel, and explores several infrastructural requirements for off-axis phase-referencing, as currently being implemented as the KI-ASTRA project. The technology of self-phasereferencing opens the way to reach very high spectral resolution in near-infrared interferometry. We present the scientific capabilities of the KI-SPR mode in detail, at the example of observations of the Be-star 48 Lib. Several spectral lines of the cirumstellar disk are resolved. We describe the first detection of Pfund-lines in an interferometric spectrum of a Be star, in addition to Br ó. The differential phase signal can be used to (i) distinguish circum-stellar line emission from the star, (ii) to directly measure line asymmetries tracing an asymetric gas density distribution, (iii) to reach a differential, astrometric precision beyond single-telescope limits sufficient for studying the radial disk structure. Our data support the existence of a radius-dependent disk density perturbation, typically used to explain slow variations of Be-disk hydrogen line profiles.Danchi, William C.; Delplancke, Françoise; Rajagopal, Jayadev K.: Optical and Infrared Interferometry II, SPIE, 77340P-77340P-6 (2010). -
Article: Constraining the evolutionary stage of class I protostars: Multiwavelength observations and modeling
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ABSTRACT: We present new Keck images at 0.9 mum and OVRO 1.3 mm continuum images of five Class I protostars in the Taurus star-forming region. We analyze these data in conjunction with broadband spectral energy distributions (SEDs) and 8-13 mum spectra from the literature using a Monte Carlo radiative transfer code. By fitting models for the circumstellar dust distributions simultaneously to the scattered light images, millimeter continuum data, and the SEDs, we attempt to distinguish between flared disks, infalling envelopes with outflow cavities, and combinations of disks and envelopes. For each of these circumstellar density distributions, we generate grids of models for varying geometries, dust masses, and accretion rates and determine the best fits by minimizing the residuals between model and data. Comparison of the residuals for best-fit disk, envelope, and disk+envelope models demonstrates that, in general, models incorporating both massive envelopes and massive embedded disks fit the imaging+SED data best. The implied envelope infall rates for these disk+envelope models are generally consistent with infall rates derived by previous investigators, although they are approximately an order of magnitude larger than inner disk accretion rates inferred from recent spectroscopic measurements. In addition, the disk masses inferred from our models are close to or larger than the limit for gravitationally stable disks, indicating that Class I disks may undergo periodic episodes of enhanced accretion, perhaps as a result of gravitational instabilities. An important caveat to these results is that in some cases, no single model can fit all of the imaging and SED data well, suggesting that further refinements to models of the circumstellar dust distributions around Class I sources are necessary. We discuss several potential improvements to the models, as well as new constraints that will become available with upcoming millimeter and infrared facilities.The Astrophysical Journal, v.635, 396-421 (2005).
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Institutions
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2008–2010
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California Institute of Technology
- • Jet Propulsion Laboratory
- • Department of Astronomy
Pasadena, CA, USA
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