Herschel HIFI observations of O${}_2$ toward Orion: special conditions for shock enhanced emission

Abstract

We report observations of molecular oxygen (O${}_2$) rotational transitions at 487 GHz, 774 GHz, and 1121 GHz toward Orion Peak A. The O2 lines at 487 GHz and 774 GHz are detected at velocities of 10-12 km/s with line widths 3 km/s; however, the transition at 1121 GHz is not detected. The observed line characteristics, combined with the results of earlier observations, suggest that the region responsible for the O${}_2$ emission is 9" (6e16 cm) in size, and is located close to the H2 Peak 1position (where vibrationally-excited H${}_2$ emission peaks), and not at Peak A, 23" away. The peak O2 column density is 1.1e18/cm2. The line velocity is close to that of 621 GHz water maser emission found in this portion of the Orion Molecular Cloud, and having a shock with velocity vector lying nearly in the plane of the sky is consistent with producing maximum maser gain along the line-of-sight. The enhanced O${}_2$ abundance compared to that generally found in dense interstellar clouds can be explained by passage of a low-velocity C-shock through a clump with preshock density 2e4/cm3, if a reasonable flux of UV radiation is present. The postshock O${}_2$ can explain the emission from the source if its line of sight dimension is ~10 times larger than its size on the plane of the sky. The special geometry and conditions required may explain why O${}_2$ emission has not been detected in the cores of other massive star-forming molecular clouds.

28 pages, 13 figures

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• A New Determination of the Binding Energy of Atomic Oxygen on Dust Grain Surfaces: Experimental Results and Simulations
• Nitrogen oxide in protostellar envelopes and shocks: the ASAI survey
• A Search for O_2 in CO-depleted Molecular Cloud Cores with Herschel
• Influence of galactic arm scale dynamics on the molecular composition of the cold and dense ISM II. Molecular oxygen abundance
• Cosmic-ray-induced dissociation and reactions in warm interstellar ices
• A First Look for Molecules between 103 and 133 MHz using the Murchison Widefield Array
• Molecular Oxygen in the nearest QSO Mrk 231
• Dust temperature and time-dependent effects in the chemistry of photodissociation regions
• Atomic oxygen abundance toward Sagittarius B2
• Self-generated ultraviolet radiation in molecular shock waves I. Effects of Lyman $\alpha$, Lyman $\beta$, and two-photon continuum