HH135/HH136 - a luminous H2 outflow towards a high-mass protostar

Context. Molecular hydrogen observations towards Herbig-Haro objects provide the possibility of studying physical processes related to star formation. Aims. Observations towards the luminous IRAS source IRAS11101-5928 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. Methods. 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. Results. 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 ± 200 K. A small fraction (0.3%) of the molecular gas is very hot, with excitation temperatures around 5500 K. 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 vs ≈ 100 km s. Electron densities of ne = 3500-4000 cm−3 are inferred from the [FeII] line ratios. Conclusions. 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.