  The formation timescale and final architecture of exoplanetary systems are
closely related to the properties of the molecular disks from which they form.
Observations of the spatial distribution and lifetime of the molecular gas at
planet-forming radii (r < 10 AU) are important for understanding the formation
and evolution of exoplanetary systems. Towards this end, we present the largest
spectrally resolved survey of H2 emission around low-mass pre-main sequence
stars compiled to date. We use a combination of new and archival
far-ultraviolet spectra from the COS and STIS instruments on the Hubble Space
Telescope to sample 34 T Tauri stars (27 actively accreting CTTSs and 7
non-accreting WTTSs) with ages ranging from roughly 1-10 Myr. We observe
fluorescent H2 emission, excited by LyA photons, in 100 of the accreting
sources, including all of the transitional disks in our sample (CS Cha, DM Tau,
GM Aur, UX Tau A, LkCa15, HD 135344B and TW Hya). The spatial distribution of
the emitting gas is inferred from spectrally resolved H2 line profiles. Some of
the emitting gas is produced in outflowing material, but the majority of H2
emission appears to originate in a rotating disk. For the disk-dominated
targets, the H2 emission originates predominately at r < 3 AU. The emission
line-widths and inner molecular radii are found to be roughly consistent with
those measured from mid-IR CO spectra.
