Detection of H_2 fluorescent emission from the DR 21 bipolar outflow

Details Detection of H_2 fluorescent emission from the DR 21 bipolar outflow Detection of H_2 fluorescent emission from the DR 21 bipolar outflow

Sep 1997

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997mnras.290..216f&link_type=abstract

Monthly Notices of the Royal Astronomical Society, Volume 290, Issue 2, pp. 216-230.

Astronomy and Astrophysics

Astronomy

22

Shock Waves, H Ii Regions, Ism: Individual: Dr 21, Ism: Jets And Outflows, Ism: Molecules, Infrared: Ism: Lines And Bands

Scientific paper

Near-infrared emission lines of molecular hydrogen (H_2) 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 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 Lyalpha 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<=G_0<=10^3 and pre-shock density n_0>=3x10^3 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.

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