Herschel HIFI Observations of O2 toward Orion: Special Conditions for Shock Enhanced Emission

We report observations of molecular oxygen (O2) 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-1 with line widths ~3 km s-1 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 O2 emission is sime9'' (6 × 1016 cm) in size, and is located close to the H 2 Peak 1 position (where vibrationally excited H2 emission peaks), and not at Peak A, 23'' away. The peak O2 column density is sime1.1 × 1018 cm-2. The line velocity is close to that of the 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 O2 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 2 × 104 cm-3, if a reasonable flux of UV radiation is present. The postshock O2 can explain the emission from the source if its line-of-sight dimension is sime10 times larger than its size on the plane of the sky. The special geometry and conditions required may explain why O2 emission has not been detected in the cores of other massive star-forming molecular clouds.