The HST/STIS Coronagraphic Survey of Pre-Main Sequence Stars

HST/STIS coronagraphically observed 14 T Tauri stars, and 11 Herbig Ae stars. Both the T Tauri and Herbig Ae star disks range from optically bright disks to non-detections which are a factor of 50-100 times fainter at r=1-3 arcsec.Three Herbig Ae disks are detected, while 5 disks and 1 envelope are seen in the T Tauri sample. Both groups comtain stars with known, large disks which are not optically detected, with the non-detections concentrated at t>5 Myr.For these protoplanetary disks, surface brightness is not correlated with bulk disk properties, indicating that we view the outer layers of optically thick disks. Disk surface brightness for both groups is correlated with transitions which are photo-excited by absorption of stellar FUV photons.These correlations suggest that the bright disks are highly flared, and that the large, dark disks are flatter. Models for Herbig Ae disks have suggested that flat disks can be produced either by shadowing by material in the inner disk, or due to grain growth and settling. Since the shadowing mechanism is expected to be less effective in T Tauri disks, and disks which have begun to centrally clear, the similarity between the Herbig Ae and T Tauri samples favors grain growth and settling. This process is likely to proceed from the inner, higher density part of the disk outward, and, in a sufficiently large sample, should result in disks with inner, dark zones, and outer flared regions. Two disks in our survey, HD 163296 and DM Tau,appear to be such transitional objects. This study is based on data from HST-GO-9136, HST-GTO-8474, HST-GTO-7065, HST-GTO-8065, and HST-GTO-9241.