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 H _ { 2 } excitation characteristics .
A theoretical H _ { 2 } 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 H _ { 2 } outflow with a total H _ { 2 } luminosity of about 2 L _ { \odot } .
The bulk of the molecular gas is characterized by a ro-vibrational excitation temperature of 2000 \pm 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 H _ { 2 } and [ FeII ] emission characteristics indicate the presence of fast , dissociative J-shocks at speeds of v _ { \mathrm { s } } \approx 100 km s ^ { -1 } .
Electron densities of n _ { \mathrm { e } } = 3500-4000 cm ^ { -3 } are inferred from the [ FeII ] line ratios .

Conclusions : The large H _ { 2 } 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 .