MIDINFRARED IMAGING OF YOUNG STELLAR OBJECTS

We present arcsecond resolution mid-infrared (8-13 mu m) images and ph otometry of four young stellar objects (YSOs)-L1551-IRS 5, HL Tau, AS 205, and AS 209 (V1121 Oph)-taken with the Berkeley Mid-Infrared Camer a. For AS 205, a known T Tauri binary, we also present near-infrared J HK images and HKL' speckle imaging data. All three single stars are un resolved in our mid-IR images, consistent with current models of the c ircumstellar material associated with these objects. Our data are the first to resolve in the mid-IR both components of the close binary AS 205 (projected separation similar to 1 ''.3 [210 AU]). Both stars are classical T Tauri stars and possess the 9.7 mu m silicate feature in e mission. AS 205 North is the IR brighter star in our data, while publi shed observations find it to be the optically fainter star. Assuming t hat the IR excesses of both components arise from circumstellar disks, we find the emitting regions (the inner few AU) of the disks to be op tically thick in the mid-IR. Pre-main-sequence evolutionary models sug gest that the AS 205 system is non-coeval; we discuss possible explana tions for this result and comment on the evolutionary status of this y oung binary. All our objects, except perhaps AS 205 South, exhibit cha nges in their mid-IR flux in measurements separated by intervals of da ys up to many years; the variations range from 30%-300%. For the class ical T Tauri stars AS 205 North and AS 209, the magnitude of the chang es seems to discount the possibility that the mid-IR variations have t he same origin as the optical and near-IR variability of T Tauri stars , namely, accretion-related features on or near the stellar photospher e. We speculate that the cause of the variability lies in the accretio n disks of these objects; the data suggest disk accretion rate fluctua tions of nearly an order of magnitude. The existence of large mid-IR v ariability argues that simultaneous multiwavelength observations are n eeded for a proper analysis of YSO spectral energy distributions.