An extremely luminous bipolar H2 flow in the DR21 star-forming region

We have mapped the spatial distribution of molecular hydrogen (H2) emission in the DR21 molecular cloud in the υ = 1−0 S(1) line at 2.12μm. The emission delineates a highly collimated, bipolar structure extending over a projected distance of ∼5pc, centred on the DR21 Hn region/molecular cloud core. This is probably the most luminous and largest galactic H2 source presently known. The extended bipolar morphology and considerable luminosity in shocked H2 line emission most probably derive from the dynamical interaction of an energetic protostellar outflow with the surrounding ambient cloud medium. The contribution of a cloud–cloud collision to the observed phenomenology is suggested as a less favourable, yet viable, alternative. Line profiles measured at the positions of peak emission in both lobes show a significant difference in both shape and width. In particular, the West lobe shows emission at blue-shifted velocities which exceed the current theoretical H2 dissociation limit. The neighbouring star-forming region W75S was similarly mapped in H2 line emission and also appears to be double-structured; it is both smaller and less luminous than the DR21 source. A supersonic mass-loss flow associated with the heavily reddened young stellar object W75 (IRS1) is proposed as the driving agent for the observed shocks in this region.