Detections of Rovibrational H2 Emission from the Disks of T Tauri Stars

We report the detection of quiescent H2 emission in the v = 1 → 0 S(1) line at 2.12183 μm in the circumstellar environment of two classical T Tauri stars, GG Tau A and LkCa 15, in high-resolution (R ≃ 60,000) spectra, bringing to four, including TW Hya and the weak-lined T Tauri star DoAr 21, the number of T Tauri stars showing such emission. The equivalent widths of the H2 emission line lie in the range 0.02-0.10 Å, and in each case the central velocity of the emission line is centered at the star's systemic velocity. The line widths range from 9 to 14 km s-1, in agreement with those expected from gas in Keplerian orbits in circumstellar disks surrounding K-type stars at distances ≥10 AU from the sources. UV fluorescence and X-ray heating are likely candidate mechanisms responsible for producing the observed emission. We present mass estimates from the measured line fluxes and show that the estimated masses are consistent with those expected from the possible mechanisms responsible for stimulating the observed emission. The high temperatures and low densities required for significant emission in the v = 1 → 0 S(1) line suggests that we have detected reservoirs of hot H2 gas located in the low-density upper atmospheres of circumstellar disks of these stars.