Detection of H2 fluorescent emission from the DR 21 bipolar outflow

Near-infrared emission lines of molecular hydrogen (H2) measured in the Kwindow (2.01 to 2.43 11m) and covering energy levels from 6000 to 24000 K show indication of different excitation conditions in the eastern and western lobes of shock-excited H2 in the DR 21 bipolar outflow. We use H2 excitation diagrams to demonstrate that neither J-nor C-type shocks can explain the observed line ratios. The higher H 2 line ratios measured for the eastern lobe are a clear indication of enhanced excitation for the high-excitation levels of the H2 molecule, which may be caused by shockproduced Lyoc resonance pumping or by direct UV excitation of H2 from the central H II region. This is consistent with the eastern lobe bordering the central H II region and therefore producing higher far-ultraviolet (FUV) fluxes. We show that the observed H2 emission can be interpreted by a simple two-component emission model consisting of a bow C-type shock which produces the low-excitation H2 emission and an FUV radiation field which produces the high-excitation emission through H2 fluorescence. The H2 line ratios are best fitted by a photodissociation region (PDR) model with parameters FUV field in the range 10 2 ::; Go::; 10 3 and preshock density n o ;;::3 x 10 3 cm-3 Using the PDR models investigated here, we suggest that in DR 21 the ortho-to-para ratio is 1.8 in the fluorescent emission component.