Herschel HIFI observations of O2toward Orion: Special conditions for shock enhanced emission

Abstract

We report observations of molecular oxygen (O2) rotational transitions at 487 GHz, 774 GHz, and 1121 GHz toward Orion Peak A. The O2lines at 487 GHz and 774 GHz are detected at velocities of 10-12 km s-1with 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 O2emission is ≃9″ (6 × 1016cm) in size, and is located close to the H 2Peak 1 position (where vibrationally excited H2emission peaks), and not at Peak A, 23″ away. The peak O2column density is ≃1.1 × 1018cm-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 O2abundance 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 × 104cm-3, if a reasonable flux of UV radiation is present. The postshock O2can 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 O2emission has not been detected in the cores of other massive star-forming molecular clouds.