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 O_2 lines at 487 GHz and 774 GHz are detected at velocities of 10-12 km s^(-1) with line widths ~3 km s^(-3); 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" (6x10^(16) cm) in size, and is located close to the H_2 Peak 1 position (where vibrationally-excited H_2 emission peaks), and not at Peak A, 23" away. The peak O_2 column density is ≃1.1x10^(18)/cm^(-2). 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 2x10^4/cm^(-3), 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.

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