2015ApJ...806..227M

Molecular oxygen (O₂) 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 H₂ Peak 1 have yielded detections of any statistical significance. The detection of the O₂N_J= 3₃-1₂and 5₄-3₄lines 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 O₂abundance, while the detection of the O₂N_J= 3₃-1₂, 5₄-3₄lines, plus the 7₆-5₆line at 1120.715 GHz, toward Orion has been ascribed to time-dependent preshock physical and chemical evolution and low-velocity (12 km/s) 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 O₂detection 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 O₂ intensities and O₂line ratios. The solution in closest agreement with the shock parameters inferred for H₂Peak 1 from other gas tracers assumes a 23 km/s shock impacting gas with a preshock density of 8x10⁴/cm3 and G₀ = 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 O₂lines (≈ 11 km/s) and recently measured H₂O 5₃₂-4₄₁maser emission at 620.701 GHz toward H₂Peak 1 suggests that the O₂emission arises behind the same shocks responsible for the maser emission, though the O₂emission 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) observed O₂ line widths despite their excitation behind a shock but also why such O₂detections are rare.