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