Herschel Observations of the T Cha Transition Disk: Constraining the
  Outer Disk Properties
  T Cha is a nearby (d = 100 pc) transition disk known to have an optically
thin gap separating optically thick inner and outer disk components. Huelamo et
al. (2011) recently reported the presence of a low-mass object candidate within
the gap of the T Cha disk, giving credence to the suspected planetary origin of
this gap. Here we present the Herschel photometry (70, 160, 250, 350, and 500
micron) of T Cha from the "Dust, Ice, and Gas in Time" (DIGIT) Key Program,
which bridges the wavelength range between existing Spitzer and millimeter data
and provide important constraints on the outer disk properties of this
extraordinary system. We model the entire optical to millimeter wavelength
spectral energy distribution (SED) of T Cha (19 data points between 0.36 and
3300 micron without any major gaps in wavelength coverage). T Cha shows a steep
spectral slope in the far-IR, which we find clearly favors models with outer
disks containing little or no dust beyond 40 AU. The full SED can be modeled
equally well with either an outer disk that is very compact (only a few AU
wide) or a much larger one that has a very steep surface density profile. That
is, T Cha's outer disk seems to be either very small or very tenuous. Both
scenarios suggest a highly unusual outer disk and have important but different
implications for the nature of T Cha. Spatially resolved images are needed to
distinguish between the two scenarios.
