Herschel HIFI observations of O$_2$ toward Orion: special conditions for shock enhanced emission

TL;DR

This study reports the detection of molecular oxygen emission in Orion, linking it to shock conditions and specific geometries that enhance O$_2$ abundance, explaining its rarity in other star-forming regions.

Contribution

It identifies the specific shock conditions and geometry that lead to enhanced O$_2$ emission in Orion, providing insights into molecular oxygen chemistry in star-forming regions.

Findings

O$_2$ detected at 487 GHz and 774 GHz, not at 1121 GHz.

O$_2$ emission region is ~9" near H2 Peak 1, not Peak A.

Enhanced O$_2$ abundance explained by low-velocity C-shock and UV radiation.

Abstract

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.