O2EMISSION TOWARD ORION H2PEAK 1 AND THE ROLE OF FUV-ILLUMINATED C-SHOCKS

Abstract

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 O2 NJ = 33?12 and 54?34 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 O2 abundance, while the detection of the O2 NJ = 33?12, 54?34 lines, plus the 76?56 line at 1120.715 GHz, toward Orion has been ascribed to time-dependent preshock physical and chemical evolution and low-velocity (12 km s?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 O2 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 O2 intensities and O2 line ratios. The solution in closest agreement with the shock parameters inferred for H2 Peak 1 from other gas tracers assumes a 23 km s?1 shock impacting gas with a preshock density of 8 ? 104 cm?3 and = 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 O2 lines (11 km s?1) and recently measured H2O 532?441 maser emission at 620.701 GHz toward H2 Peak 1 suggests that the O2 emission arises behind the same shocks responsible for the maser emission, though the O2 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?1) observed O2 line widths despite their excitation behind a shock?but also why such O2 detections are rare.