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Centro de Astrofísica da Universidade do Porto

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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