The Spitzer c2d Survey of Weak-Line T Tauri Stars III: Transitional Disks.
First Author:
Zahed Wahhaj
Email: wahhaj AT ifa.edu
Institute for Astronomy, Manoa, HI
2680 Woodlawn Drive
Honolulu, HI 96822 USA
Abstract
We present Spitzer 3.6 to 70 um photometry of 160 weak-line T Tauri stars (WTTS) in the Chamaeleon, Lupus, Ophiuchus and Taurus star formation regions, all of which are within 200 pc. For a comparative study, we also include 22 classical T Tauri stars. Spitzer sensitivities allow us to robustly detect the photosphere in IRAC bands (3.6 to 8 um) and the 24 um MIPS band. In the 70 um MIPS band, we are able to detect dust emission brighter than roughly 40 times the photosphere. These observations represent the first large WTTS survey longward of the L band, and reveal the frequency of outer disks (r > 3 AU) around WTT stars. We find a disk frequency of 40\% for on-cloud WTTS and those separated from their parent clouds by less than 1 degree in projection. However, the disk frequency for WTTS with greater separation is ~ 8% similar to value reported in the first paper in this series (Padgett et al. 2006). Cieza et al. (2007) reported that the on-cloud WTTS had an excess rate of ~ 20%. However, when we include the 70 um detections, this rate increases considerably. We find an average WTTS disk rate of 25\% when including all objects within 6 degrees of the cloud. WTTS exhibit spectral energy distributions (SED) that are very diverse, spanning the range from Class II to Class III. In fact, 10% of WTTS SEDs correspond to optically thick CTT-like disks, 15% are transitional disks with inner holes of various sizes, and 75% show no detectable dust within 20-40 AU of the star. However, our sensitivity limits do not allow us to detect disks more tenuous than the debris disk, β Pictoris (fractional luminosity = 2x10-3).
