The Herschel DIGIT Survey of Weak-line T Tauri Stars: implications for
  disk evolution and dissipation
  As part of the "Dust, Ice, and Gas In Time (DIGIT)" Herschel Open Time Key
Program, we present Herschel photometry (at 70, 160, 250, 350 and 500 micron)
of 31 Weak-Line T Tauri star (WTTS) candidates in order to investigate the
evolutionary status of their circumstellar disks. Thirteen stars in our sample
had circumstellar disks previously known from infrared observations at shorter
wavelengths, while eighteen of them had no previous evidence for a disk. We
detect a total of 15 disks as all previously known disks are detected at one or
more Herschel wavelengths and two additional disks are identified for the first
time. The spectral energy distributions (SEDs) of our targets seem to trace the
dissipation of the primordial disk and the transition to the debris disk
regime. Seven of the 15 disks appear to be optically thick primordial disks,
including two objects with SEDs indistinguishable from those of typical
Classical T Tauri stars, four objects that have significant deficit of excess
emission at all IR wavelengths, and one "pre-transitional" object with a known
gap in the disk. Despite their previous WTTS classification, we find that the
seven targets in our sample with optically thick disks show evidence for
accretion. The remaining eight disks have weaker IR excesses similar to those
of optically thin debris disks. Six of them are warm and show significant 24
micron Spitzer excesses, while the last two are newly identified cold
debris-like disks with photospheric 24 micron fluxes, but significant excess
emission at longer wavelengths. The Herschel photometry also places strong
constraints on the non-detections, where systems with F70/F70,star > 5 - 15 and
L,disk/L,star > 1xE-3 to 1xE-4 can be ruled out. We present preliminary models
for both the optically thick and optically thin disks and discuss our results
in the context of the evolution and dissipation of circumstellar disks.
