ISO observations of molecular hydrogen in the DR 21 bipolar outflow

We demonstrate that a wide range of molecular hydrogen excitation can be observed in protostellar outflows at wavelengths in excess of 5 ?m. Cold H2 in DR 21 is detected through the pure rotational transitions in the ground vibrational level (0-0). Hot H2 is detected in pure rotational transitions within higher vibrational levels (1-1, 1-2, etc.). Although this emission is relatively weak, we have detected two 1-1 lines in the DR 21 outflow with the ISO SWS instrument. We thus investigate molecular excitation over energy levels corresponding to the temperature range 1015-15 722 K, without the uncertainty introduced by differential extinction when employing near-infrared data. This gas is thermally excited. We uncover a rather low H2 excitation in the DR 21 West Peak. The line emission cannot be produced from single C-shocks or J-shocks; a range of shock strengths is required. This suggests that bow shocks and/or bow-generated supersonic turbulence is responsible. We are able to distinguish this shock-excited gas from the fluoresced gas detected in the K band, providing support for the dual-excitation model of Fernandes, Brand & Burton.