A survey for molecular hydrogen rotational emission in low-mass pre-main sequence stars

We have used ISO/SWS to search for molecular hydrogen S(0) and S(1) rotational emission toward 14 low mass pre-main sequence stars located in the nearby Chamaeleon, Ophiuchus, and Taurus-Auriga star forming regions. Eight classical T Tauri stars and six weak line T Tauri stars were observed. The survey was motivated by model calculations which indicate that there should be enough warm gas in a circumstellar/protoplanetary disk to produce H-2 emission detectable by ISO. Emission was strongly detected toward the weak line T Tauri star SR 9, weakly detected toward three other targets, and not detected at all in the remaining 8 objects. For SR 9, the observed rotational temperature is 120 K. At this temperature, the observed H-2 line fluxes suggest a circumstellar gas mass comparable to that inferred from the millimeter continuum dust emission from this source. For the other three sources only the S(1) H-2 line was detected, and thus it is difficult to draw strong conclusions about the emission mechanism for them. These ISO results suggest that broader, more sensitive surveys with SOFIA or perhaps with SIRTF should be able to address the question of the gas dissipation timescale in a protoplanetary disk.