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 Ðtted 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 signiÐcant vertical settling of photospheric grains. While this work furnishes further evidence that passive reprocessing of starlight by Ñared 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.

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 dependence on luminosity and effective temperature. For most targets, the measured near-infrared sizes (ranging from 0.2 to 4 AU ) support a simple disk model possessing a central optically thin (dust-free) cavity, ringed by hot dust emitting at the expected sublimation temperatures (T s $ 1000-1500 K). Furthermore, we find a tight correlation of disk size with 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.

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We report novel, high-angular resolution interferometric measurements that imply 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-meter 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 AGN unification models invoking hot, optically thick dust may not be applicable to NGC 4151.

We present circumstellar disk models for two preÈmain-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, Ñat-disk model. The model adequately Ðts the data for SU Aur but not for T Tau. We Ðnd that there are signiÐcant di †erences in the physical parameters suggested by the individual data sets. The size of the inner disk radius as implied by the infrared interferometry data (Dtenths of AU) is larger than expected for a Ñat-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.

We present observations of the Galactic Cepheids Aql and 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 Aql of 320 AE 32 pc and a distance to Gem of 362 AE 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.

We present infrared observations of four young stellar objects 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 PTI's nominal fringe spacing of 4 milliarcsec (mas), while the emission region of AB Aur is over-resolved 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 AU in size for T Tau and SU Aur and ∼1 AU for MWC 147.Subject headings: stars:pre-main sequence, circumstellar matter 1 to be published in the Astrophysical Journal

We present the technique and results of a survey of stars within 8 pc of the Sun with declinations d [ [35¡ (J2000.00). The survey, designed to Ðnd without color bias faint companions, consists of optical coronagraphic images of the 1@ Ðeld of view centered on each star and infrared direct images with a 32A Ðeld of view. The images were obtained through the optical Gunn r and z Ðlters and the infrared J and K Ðlters. 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 identiÐed 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 di †erent masses and ages.