Michelle J. Creech-Eakman

Spectral Energy Distributions of Passive T Tauri and Herbig Ae Disks: Grain Mineralogy, Parameter Dependences, and Comparison with Infrared Space Observatory LWS Observations

We improve upon the radiative, hydrostatic equilibrium models of passive circumstellar disks constructed by Chiang & Goldreich. New features include (1) an account for a range of particle sizes, (2) employment of laboratory-based optical constants of representative grain materials, and (3) numerical solution of the equations of radiative and hydrostatic equilibrium within the original two-layer (disk surface plus disk interior) approximation. We systematically explore how the spectral energy distribution (SED) of a face-on disk depends on grain size distributions, disk geometries and surface densities, and stellar photospheric temperatures. Observed SEDs of three Herbig Ae and two T Tauri stars, including spectra from the Long Wavelength Spectrometer (LWS) aboard the Infrared Space Observatory (ISO), are fitted with our models. Silicate emission bands from optically thin, superheated disk surface layers appear in nearly all systems. Water ice emission bands appear in LWS spectra of two of the coolest stars. Infrared excesses in several sources are consistent with significant vertical settling of photospheric grains. While this work furnishes further evidence that passive reprocessing of starlight by flared disks adequately explains the origin of infrared-to-millimeter wavelength excesses of young stars, we emphasize by explicit calculations how the SED alone does not provide sufficient information to constrain particle sizes and disk masses uniquely.

The near-infrared size-luminosity relations for Herbig Ae/Be disks

We report the results of a sensitive K-band survey of Herbig Ae/Be disk sizes using the 85 m baseline Keck Interferometer. Targets were chosen to span the maximum range of stellar properties to probe the disk size dependenceonluminosityandeffectivetemperature.Formosttargets,themeasurednear-infraredsizes(rangingfrom0.2to 4AU)supportasimple diskmodelpossessingacentralopticallythin(dust-free) cavity,ringedbyhotdustemitting at theexpected sublimation temperatures (Ts � 1000–1500 K).Furthermore, wefindatightcorrelation of disksizewith source luminosity R / L 1 =2 for Ae and late Be systems (valid over more than two decades in luminosity), confirming earlier suggestions based on lower quality data. Interestingly, the inferred dust-free inner cavities of the highest luminosity sources (Herbig B0–B3 stars) are undersized compared to predictions of the ‘‘optically thin cavity’’ model, likely because of optically thick gas within the inner AU. Subject headingg accretion, accretion disks — circumstellar matter — instrumentation: interferometers — radiative transfer — stars: formation — stars: pre–main-sequence

CONSTRAINTS ON CIRCUMSTELLAR DISK PARAMETERS FROM MULTIWAVELENGTH OBSERVATIONS: T TAURI AND SU AURIGAE

We present circumstellar disk models for two pre-main-sequence objects, T Tau and SU Aur. The models are based on interferometric data from infrared and millimeter wavelengths and infrared photometry from the literature. The physical properties of the disk are examined by calculating parameter probabilities based on a passive, flat-disk model. The model adequately fits the data for SU Aur but not for T Tau. We find that there are significant differences in the physical parameters suggested by the individual data sets. The size of the inner disk radius as implied by the infrared interferometry data (~tenths of AU) is larger than expected for a flat-disk model. This discrepancy is discussed in consideration of more complex disk models that include the presence of a hot, inner region or wall in the disk.

Interferometer Observations of Subparsec-Scale Infrared Emission in the Nucleus of NGC 4151

We report novel, high angular resolution interferometric measurements that imply that the near-infrared nuclear emission in NGC 4151 is unexpectedly compact. We have observed the nucleus of NGC 4151 at 2.2 μm using the two 10 m Keck telescopes as an interferometer and find a marginally resolved source ≤0.1 pc in diameter. Our measurements rule out models in which a majority of the K-band nuclear emission is produced on scales larger than this size. The interpretation of our measurement most consistent with other observations is that the emission mainly originates directly in the central accretion disk. This implies that active galactic nucleus unification models invoking hot, optically thick dust may not be applicable to NGC 4151.

A Coronagraphic Survey for Companions of Stars within 8 Parsecs

We present the technique and results of a survey of stars within 8 pc of the Sun with declinations δ > -35° (J2000.00). The survey, designed to find without color bias faint companions, consists of optical coronagraphic images of the 1 field of view centered on each star and infrared direct images with a 32 field of view. The images were obtained through the optical Gunn r and z filters and the infrared J and K filters. The survey achieves sensitivities up to 4 absolute magnitudes fainter than the prototype brown dwarf, Gliese 229B. However, this sensitivity varies with the seeing conditions, the intrinsic brightness of the star observed, and the angular distance from the star. As a result, we tabulate sensitivity limits for each star in the survey. We used the criterion of common proper motion to distinguish companions and to determine their luminosities. In addition to the brown dwarf Gl 229B, we have identified six new stellar companions of the sample stars. Since the survey began, accurate trigonometric parallax measurements for most of the stars have become available. As a result, some of the stars we originally included should no longer be included in the 8 pc sample. In addition, the 8 pc sample is incomplete at the faint end of the main sequence, complicating our calculation of the binary fraction of brown dwarfs. We assess the sensitivity of the survey to stellar companions and to brown dwarf companions of different masses and ages.

INFRARED INTERFEROMETRIC OBSERVATIONS OF YOUNG STELLAR OBJECTS

We present infrared observations of four young stellar objects made using the Palomar Testbed Interferometer (PTI). For three of the sources, T Tau, MWC 147, and SU Aur, the 2.2 μm emission is resolved at the PTIs nominal fringe spacing of 4 mas, while the emission region of AB Aur is overresolved on this scale. We fit the observations with simple circumstellar material distributions and compare our data to the predictions of accretion disk models inferred from spectral energy distributions. We find that the infrared emission region is tenths of an AU in size for T Tau and SU Aur and ~1 AU for MWC 147.

Multiepoch Interferometric Study of Mira Variables. I. Narrowband Diameters of RZ Pegasi and S Lacertae

As part of the long-term monitoring of Mira variables at the Palomar Testbed Interferometer, we report high-resolution narrowband angular sizes of the oxygen-rich Mira S Lac and the carbon-rich Mira RZ Peg in the near-infrared. The data set spans three pulsation cycles for S Lac and two pulsation cycles for RZ Peg (a total of 1070 25 s observations) and represents the first study to correlate multiepoch narrowband interferometric data of Mira variables. When the calibrated visibility data are fitted using a uniform disk brightness model, differences are seen in their angular diameters as a function of wavelength within the K band (2.0-2.4 μm), the source of which is believed to be molecular absorptions in or above the photospheres of the two chemically different Miras. Using visible photometric data provided by the Association Francaise des Observateurs dEtoiles Variables (AFOEV), the continuum minimum size of RZ Peg lags this by 0.28 ± 0.02 in pulsation, similar to the phase lag found in Correlation Radial Velocities (CORAVEL) data. However, for S Lac, the continuum minimum size tracks the visual maximum brightness. Based on the mean of the continuum angular diameter cycloids, basic stellar parameters are computed for both RZ Peg and S Lac, with both showing maximum atmospheric extension with respect to the 2.0 and 2.4 μm diameters near phase 0.9. Using the mean value of the fitted cycloids, RZ Peg has a radius Rmean = 377 ± 111 R☉ and a mean Teff = 2706 ± 36 K; S Lac has a radius Rmean = 292 ± 73 R☉ and a mean Teff = 2605 ± 47 K. The dominant source of error in the radii is the large uncertainty in the distances to these two stars.

Long-Baseline Interferometric Observations of Cepheids

We present observations of the Galactic Cepheids η Aql and ζ Gem. Our observations are able to resolve the diameter changes associated with pulsation. This allows us to determine the distance to the Cepheids independent of photometric observations. We determine a distance to η Aql of 320 ± 32 pc and a distance to ζ Gem of 362 ± 38 pc. These observations allow us to calibrate surface brightness relations for use in extragalactic distance determination. They also provide a measurement of the mean diameter of these Cepheids, which is useful in constructing structural models of this class of star.

Keck Interferometer Observations of FU Orionis Objects

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.

Angular Size Measurements of Mira Variable Stars at 2.2 Microns. II.

We present angular size measurements of 22 oxygen-rich Mira variable stars. These data are part of a long-term observational program using the Infrared Optical Telescope Array to characterize the observable behavior of these stars. Complementing the infrared angular size measurements, values for variable star phase, spectral type, bolometric flux, and distance were established for stars in the sample; flux and distance led to values for effective temperature (Teff) and linear radius, respectively. Additionally, values for the K-[12] color excess were established for these stars, which is indicative of dusty mass loss. Stars with higher color excess are shown to be systematically 120 R⊙ larger than their low color excess counterparts, regardless of period. This analysis appears to present a solution to a long-standing question presented by the evidence that some Mira angular diameters are indicative of first-overtone pulsation, while other diameters are more consistent with fundamental pulsation. A simple examination of the resultant sizes of these stars in the context of pulsation mode is consistent with at least some of these objects pulsating in the fundamental mode.