Spatially Resolved Circumstellar Structure of Herbig Ae/Be Stars in the Near-Infrared

We have conducted the first systematic study of Herbig Ae/Be stars using the technique of long baseline stellar interferometry in the near-infrared, with the objective of characterizing the distribution and properties of the circumstellar dust responsible for the excess near-infrared fluxes from these systems. The observations for this work have been conducted at the Infrared Optical Telescope Array (IOTA). The principal result of this paper is that the interferometer resolves the source of infrared excess in 11 of the 15 systems surveyed. A new binary, MWC 361-A, has been detected interferometrically for the first time. The visibility data for all the sources has been interpreted within the context of four simple models which represent a range of plausible representations for the brightness distribution of the source of excess emission: a Gaussian, a narrow uniform ring, a flat blackbody disk with a single temperature power law, and an infrared companion. We find that the characteristic sizes of the near-infrared emitting regions are larger than previously thought (0.5-5.9 AU, as given by the FWHM of the Gaussian intensity). A further major result of this paper is that the sizes measured, when combined with the observed spectral energy distributions, essentially rule out accretion disk models represented by blackbody disks with the canonical T(r) ∝ r^-3/4 law. We also find that, within the range observed in this study, none of the sources (except the new binary) shows varying visibilities as the orientation of the interferometer baseline changes. This is the expected behavior for sources which appear circularly symmetric on the sky, and for the sources with the largest baseline position angle coverage (AB Aur, MWC 1080-A) asymmetric brightness distributions (such as inclined disks or binaries) become highly unlikely. Taken as an ensemble, with no clear evidence in favor of axisymmetric structure, the observations favor the interpretation that the circumstellar dust is distributed in spherical envelopes (the Gaussian model) or thin shells (the ring model). This interpretation is also supported by the result that the measured sizes, combined with the excess near-infrared fluxes, imply emission of finite optical depth, as required by the fact that the central stars are optically visible. The measured sizes and brightnesses do not correlate strongly with the luminosity of the central star. Moreover, in two cases, the same excess is observed from circumstellar structures that differ in size by more than a factor of 2 and surround essentially identical stars. Therefore, different physical mechanisms for the near-infrared emission may be at work in different cases, or alternatively, a single underlying mechanism with the property that the same infrared excess is produced on very different physical scales.