Residual Gas & Dust Around Transition Objects and Weak T Tauri Stars
  Residual gas in disks around young stars can spin down stars, circularize the
orbits of terrestrial planets, and whisk away the dusty debris that is expected
to serve as a signpost of terrestrial planet formation. We have carried out a
sensitive search for residual gas and dust in the terrestrial planet region
surrounding young stars ranging in age from a few Myr to ~10 Myr in age. Using
high resolution 4.7 micron spectra of transition objects and weak T Tauri
stars, we searched for weak continuum excesses and CO fundamental emission,
after making a careful correction for the stellar contribution to the observed
spectrum. We find that the CO emission from transition objects is weaker and
located further from the star than CO emission from non-transition T Tauri
stars with similar stellar accretion rates. The difference is possibly the
result of chemical and/or dynamical effects (i.e., a low CO abundance or
close-in low-mass planets). The weak T Tauri stars show no CO fundamental
emission down to low flux levels (5 x 10^(-20) - 10^{-18} W/m^2). We illustrate
how our results can be used to constrain the residual disk gas content in these
systems and discuss their potential implications for star and planet formation.
