Detection of quiescent molecular hydrogen gas in the circumstellar disk ofa classical T Tauri star

We have detected emission from the quiescent, molecular hydrogen gas in the disk around the X-ray bright, classical T Tauri star TW Hya in the 1-0 S(1 ) line at 2.1218 mu m through high-resolution spectroscopy using CSHELL on NASA's Infrared Telescope Facility. The brightness of the observed emission line is consistent with that predicted from models for X-ray excitation of the 1-0 S(1) line of H-2. This result demonstrates that X-ray ionization i s a plausible mechanism for excitation of the H-2 in the gaseous disks of T Tauri stars. In addition, from these high-resolution spectra we have measu red the radial velocities of three T Tauri stars in the TW Hya association, CD -33 degrees 7795, HD 98800, and TW Hya. We find that the radial velocit ies of these three stars are very similar to each other and to those of at least three other stars presumed to be members of the TW Hya association, H en 600A, Hen 600B, and CD -29 degrees 8887. This result, combined with the similar proper motions of all six of these stars, lends support to the hypo thesis that all of these stars share a common origin in a now-dispersed mol ecular cloud.