Detection of H2 fluorescent emission from the DR 21 bipolar outflow
A. J. L. Fernandes, P. W. J. L. Brand, M. G. Burton
Abstract
Near-infrared emission lines of molecular hydrogen (H2) measured in the K window (2.01 to 2.43 μm) 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 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 H2 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 shock-produced Lyalpha 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 102≤G0≤103 and pre-shock density n0≥3x103 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.
Monthly Notices of the Royal Astronomical Society
Volume 290, Page 216
September 1997
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