2006A&A...457..157G

Molecular hydrogen observations towards Herbig-Haro objects provide the possibility of studying physical processes related to star formation. Observations towards the luminous IRAS source IRAS 11101-5829 and the associated Herbig-Haro objects HH135/HH136 are obtained to understand whether high-mass stars form via the same physical processes as their low-mass counterparts. Near-infrared imaging and spectroscopy are used to infer H₂ excitation characteristics. A theoretical H₂ spectrum is constructed from a thermal ro-vibrational population distribution and compared to the observations. The observations reveal the presence of a well-collimated, parsec-sized H₂ outflow with a total H₂ luminosity of about 2L_☉. The bulk of the molecular gas is characterized by a ro-vibrational excitation temperature of 2000±200K. A small fraction (0.3%) of the molecular gas is very hot, with excitation temperatures around 5500K. The molecular emission is associated with strong [FeII] emission. The H₂ and [FeII] emission characteristics indicate the presence of fast, dissociative J-shocks at speeds of v_s≃100km/s. Electron densities of n_e=3500-4000cm^–3 are inferred from the [FeII] line ratios. The large H₂ luminosity combined with the very large source luminosity suggests that the high-mass protostar that powers the HH135/HH136 flow forms via accretion, but with a significantly increased accretion rate compared to that of low-mass protostars.