<i>HERSCHEL</i> HIFI OBSERVATIONS OF O₂ TOWARD ORION: SPECIAL CONDITIONS FOR SHOCK ENHANCED EMISSION

Date

2014-10-01

Author

Chen, Jo-Hsin
Goldsmith, Paul F.
Viti, Serena
Snell, Ronald
Lis, Dariusz C.
Benz, Arnold
Bergin, Edwin
Black, John
Caselli, Paola
Encrenaz, Pierre
Falgarone, Edith
Goicoechea, Javier R.
Hjalmarson, Ake
Hollenbach, David
Kaufman, Michael
Melnick, Gary
Neufeld, David
Pagani, Laurent
van der Tak, Floris
van Dishoeck, Ewine
Yıldız, Umut

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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 similar to 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 O-2 emission is similar or equal to 9" (6 x 10(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 similar to 1.1 x 10(18) 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 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 2 x 10(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 similar or equal to 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.

URI

https://hdl.handle.net/11511/115744

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Department of Physics, Article