2010ApJ...725.1100G

We present new observations of near-infrared molecular hydrogen (H₂) line emission in a sample of 18 Class I and flat-spectrum low-mass protostars, primarily in the Tau-Aur and ρ Oph dark clouds. The line emission is extended by up to several arcseconds (several hundred AU) for most objects, and there is little night-to-night variation in line strength coincident with the continuum point source. Flux ratios of H₂ v = 2-1 S(1) and v = 1-0 S(1) lines are consistent with this emission arising in jets or winds in many objects. However, most objects have only small offsets (under 10 km/s) between their H₂ and photospheric radial velocities. No objects have line ratios which are clearly caused solely by UV excitation, but the H₂emission of several objects may be caused by UV or X-ray excitation in the presence of circumstellar dust. There are several objects in the sample whose observed velocities and line fluxes suggest quiescent, non-mechanical origins for their molecular hydrogen emissions. Overall, we find the H₂ emission properties of these protostars to be similar to the T Tauri stars studied in previous surveys.