%0 Journal Article
%T Herschel HIFI observations of O$_2$ toward Orion: special conditions for shock enhanced emission
%A Jo-Hsin Chen
%A Paul F. Goldsmith
%A Serena Viti
%A Ronald Snell
%A Dariusz C. Lis
%A Arnold Benz
%A Edwin Bergin
%A John Black
%A Paola Caselli
%A Pierre Encrenaz
%A Edith Falgarone
%A Javier R. Goicoechea
%A Ake Hjalmarson
%A David Hollenbach
%A Michael Kaufman
%A Gary Melnick
%A David Neufeld
%A Laurent Pagani
%A Floris van der Tak
%A Ewine van Dishoeck
%A Umut A. Yildiz
%J Physics
%D 2014
%I arXiv
%R 10.1088/0004-637X/793/2/111
%X We report observations of molecular oxygen (O$_2$) 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 O$_2$ emission is 9" (6e16 cm) in size, and is located close to the H2 Peak 1position (where vibrationally-excited H$_2$ emission peaks), and not at Peak A, 23" away. The peak O2 column density is 1.1e18/cm2. 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 2e4/cm3, 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.
%U http://arxiv.org/abs/1408.1962v1