Mid-infrared imaging of young stellar objects

We present arcsecond resolution mid-infrared (8-13 mu m) images and photometry of four young stellar objects (YSOs)-L1551-IRS 5, HL Tau, AS 205, and AS 209 (V1121 Oph)-taken with the Berkeley Mid-Infrared Camera. For AS 205, a known T Tauri binary, we also present near-infrared JHK images and HKL' speckle imaging data. All three single stars are unresolved in our mid-IR images, consistent with current models of the circumstellar 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 emission. AS 205 North is the IR brighter star in our data, while published observations find it to be the optically fainter star. Assuming that the IR excesses of both components arise from circumstellar disks, we find the emitting regions (the inner few AU) of the disks to be optically thick in the mid-IR. Pre-main-sequence evolutionary models suggest that the AS 205 system is non-coeval; we discuss possible explanations for this result and comment on the evolutionary status of this young binary. All our objects, except perhaps AS 205 South, exhibit changes in their mid-IR flux in measurements separated by intervals of days up to many years; the variations range from 30%-300%. For the classical T Tauri stars AS 205 North and AS 209, the magnitude of the changes seems to discount the possibility that the mid-IR variations have the same origin as the optical and near-IR variability of T Tauri stars, namely, accretion-related features on or near the stellar photosphere. We speculate that the cause of the variability lies in the accretion disks of these objects; the data suggest disk accretion rate fluctuations of nearly an order of magnitude. The existence of large mid-IR variability argues that simultaneous multiwavelength observations are needed for a proper analysis of YSO spectral energy distributions.