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K. Stapelfeldt (Jet Propulsion Laboratory, Caltech), D. Padgett (SIRTF Science Center, Caltech), J. Krist (Space Telescope Science Institute), F. Menard (Canada-France-Hawaii Telescope)
We report on the first extensive deep imaging survey of nearby T Tauri stars using HST. The sample consists of 120 young stars with distances \le 160 parsecs. All of the targets possess millimeter continuum emission suggesting the presence of circumstellar disks, and/or optical polarization potentially indicating the presence of compact reflection nebulosity. A wide variety of circumstellar nebulae are revealed in this data set. Most interesting are eight sources where HST directly detects disks in scattered light. These include edge-on disks in the HK Tau and HV Tau binary systems; the circumbinary ring in UY Aur; intermediate inclination disks associated with GM Aur, GO Tau, Sz 82, and DoAr 25; and a face-on disk around TW Hya. The disk sizes are comparable to or larger than our solar system's Kuiper Belt. Modeling of the HK Tau and TW Hya disks indicate there is no major depletion of micron-sized dust grains in the outer regions of these disks, and thus any planet formation process must still be in its earliest stages.
For the majority of the targets observed, however, HST/WFPC2 detects no nebulosity at all: only the stellar point spread function is seen. This includes more than 15 objects where millimeter interferometers have clearly resolved gaseous disks larger than 3 arcsec in diameter. Adaptive optics surveys have also failed to detect the scattered light from a similar but smaller sample disks surveyed to date. Several possibilities are available to explain why these disks have gone undetected, including unfavorable orientation, low dust albedo, or a highly flattened geometry. Continued progress in optical/near-IR disk imaging will require better high contrast imaging performance than has been available with current instrumentation.
This work is supported by NASA under Space Telescope General Observer grants 7387 and 8216.