O{sub 2} emission toward Orion H{sub 2} peak 1 and the role of FUV-illuminated C-shocks
- Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MS 66, Cambridge, MA 02138 (United States)
- Department of Physics and Astronomy, San José State University, San Jose, CA 95192 (United States)
Molecular oxygen (O{sub 2}) has been the target of ground-based and space-borne searches for decades. Of the thousands of lines of sight surveyed, only those toward Rho Ophiuchus and Orion H{sub 2} Peak 1 have yielded detections of any statistical significance. The detection of the O{sub 2} N{sub J} = 3{sub 3}–1{sub 2} and 5{sub 4}–3{sub 4} lines at 487.249 GHz and 773.840 GHz, respectively, toward Rho Ophiuchus has been attributed to a short-lived peak in the time-dependent, cold-cloud O{sub 2} abundance, while the detection of the O{sub 2} N{sub J} = 3{sub 3}–1{sub 2}, 5{sub 4}–3{sub 4} lines, plus the 7{sub 6}–5{sub 6} line at 1120.715 GHz, toward Orion has been ascribed to time-dependent preshock physical and chemical evolution and low-velocity (12 km s{sup −1}) non-dissociative C-type shocks, both of which are fully shielded from far-ultraviolet (FUV) radiation, plus a postshock region that is exposed to an FUV field. We report a re-interpretation of the Orion O{sub 2} detection based on new C-type shock models that fully incorporate the significant effects the presence of even a weak FUV field can have on the preshock gas, shock structure, and postshock chemistry. In particular, we show that a family of solutions exists, depending on the FUV intensity, that reproduces both the observed O{sub 2} intensities and O{sub 2} line ratios. The solution in closest agreement with the shock parameters inferred for H{sub 2} Peak 1 from other gas tracers assumes a 23 km s{sup −1} shock impacting gas with a preshock density of 8 × 10{sup 4} cm{sup −3} and G{sub o} = 1, substantially different from that inferred for the fully shielded shock case. As pointed out previously, the similarity between the LSR velocity of all three O{sub 2} lines (≈11 km s{sup −1}) and recently measured H{sub 2}O 5{sub 32}–4{sub 41} maser emission at 620.701 GHz toward H{sub 2} Peak 1 suggests that the O{sub 2} emission arises behind the same shocks responsible for the maser emission, though the O{sub 2} emission is almost certainly more extended than the localized high-density maser spots. Since maser emission arises along lines of sight of low-velocity gradient, indicating shock motion largely perpendicular to our line of sight, we note that this geometry can explain not only the narrow (≲3 km s{sup −1}) observed O{sub 2} line widths despite their excitation behind a shock but also why such O{sub 2} detections are rare.
- OSTI ID:
- 22883013
- Journal Information:
- Astrophysical Journal, Vol. 806, Issue 2; Other Information: Country of input: International Atomic Energy Agency (IAEA); Since 2009, the country of publication for this journal is the UK.; ISSN 0004-637X
- Country of Publication:
- United Kingdom
- Language:
- English
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