HERSCHEL HIFI OBSERVATIONS OF O 2 TOWARD ORION: SPECIAL CONDITIONS FOR SHOCK ENHANCED EMISSION

• Published in: The Astrophysical journal Vol. 793; no. 2; p. 111
• Main Authors: 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, Åke, Hollenbach, David, Kaufman, Michael, Melnick, Gary, Neufeld, David, Pagani, Laurent, van der Tak, Floris, van Dishoeck, Ewine, Yıldız, Umut A.
• Format: Journal Article
• Language: English
• Published: American Astronomical Society 01.10.2014
• Subjects: aperture synthesis; Astrophysics; cologne database; hot core; interstellar molecular clouds; line survey; massive stars; Physics; rho-ophiuchi; star-formation; temporal evolution; wave-astronomy-satellite
• ISSN: 1538-4357; 0004-637X; 1538-4357
• DOI: 10.1088/0004-637X/793/2/111

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.