SIMBAD references

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 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 O₂emission is ≃9'' (6x10¹⁶ cm) in size, and is located close to the H₂Peak 1 position (where vibrationally excited H₂emission peaks), and not at Peak A, 23'' away. The peak O₂column density is ≃1.1x10¹⁸/cm2. 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 2x10⁴/cm3, 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 ≃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.