Detection of H-2 fluorescent emission from the DR21 bipolar outflow

Near-infrared emission lines of molecular hydrogen (H-2) measured in the K window (2.01 to 2.43 mu m) and covering energy levels from 6000 to 24 000 K show indication of different excitation conditions in the eastern and western lobes of shock-excited H-2 in the DR 21 bipolar outflow. We use H-2 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 H-2 molecule, which may be caused by shock-produced Ly alpha resonance pumping or by direct UV excitation of H-2 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 H-2 emission can be interpreted by a simple two-component emission model consisting of a bow C-type shock which produces the low-excitation H-2 emission and an FUV radiation field which produces the high-excitation emission through H-2 fluorescence, The H-2 line ratios are best fitted by a photodissociation region (PDR! model with parameters FUV field in the range 10(2) less than or equal to G(0) less than or equal to 10(3) and preshock density n(0) greater than or equal to 3 x 10(3) cm(-3). Using the PDR models investigated here, we suggest that in DR21 the ortho-to-para ratio is 1.8 in the fluorescent emission component.