2008ApJ...688.1326B

We report the results of a search for pure rotational molecular hydrogen emission from the circumstellar environments of young stellar objects with disks using the Texas Echelon Cross Echelle Spectrograph (TEXES) on the NASA Infrared Telescope Facility and the Gemini North Observatory. We searched for mid-infrared H₂ emission in the S(1), S(2), and S(4) transitions. Keck/NIRSPEC observations of the H₂S(9) transition were included for some sources as an additional constraint on the gas temperature. We detected H₂ emission from 6 of 29 sources observed: AB Aur, DoAr 21, Elias 29, GSS 30 IRS 1, GV Tau N, and HL Tau. Four of the six targets with detected emission are class I sources that show evidence for surrounding material in an envelope in addition to a circumstellar disk. In these cases, we show that accretion shock heating is a plausible excitation mechanism. The detected emission lines are narrow (∼10 km/s), centered at the stellar velocity, and spatially unresolved at scales of 0.4", which is consistent with origin from a disk at radii 10-50 AU from the star. In cases where we detect multiple emission lines, we derive temperatures ≳500 K from ∼1 M_⊕ of gas. Our upper limits for the nondetections place upper limits on the amount of H₂ gas with T>500 K of less than a few Earth masses. Such warm gas temperatures are significantly higher than the equilibrium dust temperatures at these radii, suggesting that the gas is decoupled from the dust in the regions that we are studying and that processes such as UV, X-ray, and accretion heating may be important.