DETECTION OF H-2 FLUORESCENT EMISSION FROM THE DR21 BIPOLAR OUTFLOW

Near-infrared emission lines of molecular hydrogen (H-2) measured in t he K window (2.01 to 2.43 mu m) and covering energy levels from 6000 t o 24 000 K show indication of different excitation conditions in the e astern and western lobes of shock-excited H-2 in the DR 21 bipolar out flow. We use H-2 excitation diagrams to demonstrate that neither J- no r C-type shocks can explain the observed line ratios, The higher H-2 l ine ratios measured for the eastern lobe are a clear indication of enh anced excitation for the high-excitation levels of the H-2 molecule, w hich 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 cons istent with the eastern lobe bordering the central H II region and the refore producing higher far-ultraviolet (FUV) fluxes. We show that the observed H-2 emission can be interpreted by a simple two-component em ission model consisting of a bow C-type shock which produces the low-e xcitation H-2 emission and an FUV radiation field which produces the h igh-excitation emission through H-2 fluorescence, The H-2 line ratios are best fitted by a photodissociation region (PDR! model with paramet ers 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 th at in DR21 the ortho-to-para ratio is 1.8 in the fluorescent emission component.