AAS 197, January 2001
Session 91. Disks around YSOs
Oral, Wednesday, January 10, 2001, 10:30am-12:00noon, Pacific One

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[91.02] The Origin of the Infrared Excess of Herbig Stars

E. F. Polomski, C. M. Telesco (University of Florida)

The recent discoveries of extrasolar planets have spurred investigations of pre-main-sequence stars and their environments. Many young stars exhibit large excesses of infrared emission that have been attributed to circumstellar disks. In the case of the the T Tauri and Vega class stars, the association of the IR excess with a disk has been confirmed by direct imaging. However, no disks have been convincingly detected around the intermediate mass stars, the Herbig Ae/Be stars. This research presents for the first time a high spatial resolution mid-IR study of the circumstellar environments of the Herbig stars.

I find that the mid-IR emission of Ae/Be stars is not confined to an optically thick disk, but originates in a number of physical processes. In a survey of 21 fields, I find numerous embedded companions as well as large scale extended emission around seven objects. Over half the sample, 12/21 fields, shows significant excesses related to either companions or large-scale extended emission. The extended emission shows a wide range of morphologies, with only one object, a deeply embedded companion to LkH\alpha 234, possibly showing disk-like structure.

Revised spectral indices which exclude emission from companions or envelopes, are much flatter than expected from a geometrically thin, optically thick circumstellar disk. A histogram of the 3/10 \mum spectral indices indicates that they peak outside the regime expected for the classical disk model. However, the broad peak does encompass the spectral indices expected from flared disk models.

My observations do not conclusively rule out circumstellar disks for any of the objects in my sample. Disks could still exist within the unresolved cores of the images; however these observations do place limits on disk sizes and constrain disk models. Clearly, the dusty environments of Ae/Be stars are more complicated than previously assumed. High spatial resolution observations are essential to reveal the origin of the emission and the geometry of the dust in these environments.

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