HERSCHELHIFI OBSERVATIONS OF O₂TOWARD ORION: SPECIAL CONDITIONS FOR SHOCK ENHANCED EMISSION

Abstract

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 ∼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 9 (6 × 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 1.1 × 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 × 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 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.

Search for HOOH in Orion

“…The quoted line widths (FWHMs) are only lower limits because of the difficulty in accurately determining the local continuum on sloping backgrounds. These HOOH widths are smaller than those for O 2 reported by Chen et al (2014). The 252 GHz line was not detected.…”

“…Goldsmith et al 2000;Pagani et al 2003), with the definite detection of the molecule in merely two sources, viz. ρ Oph A (Larsson et al 2007;Liseau et al 2012) and Orion A (Goldsmith et al 2011;Chen et al 2014). Some cases have been either resolved or remained undecided (e.g.…”

The complexity of Orion: an ALMA view

“…(Goldsmith et al 2000) and Odin (Pagani et al 2003) spacecrafts, counterbalanced by a single detection in ρ Ophiuchi (Larsson et al 2007) with Odin, we detected O 2 towards Orion (Goldsmith et al 2011) and confirmed the detection towards ρ Ophiuchi, (Liseau et al 2012). However, the location of the emission in Orion is ill-defined and that made the interpretation of the observations difficult (Goldsmith et al 2011;Chen et al 2014;Melnick & Kaufman 2015). This is despite the rare velocity value of 11.3 km s −1 , which should have limited the probable emission region to a few spots at the same LSR velocity as traced by the NH 3 (6, 6) inversion transition (Goddi et al 2011).…”

“…Two pointing directions were observed, the same as those used in the Herschel observations (Goldsmith et al 2011;Chen et al 2014), namely the KL region, RA J2000 : 05 h 35 m 14 s .160, Dec J2000 : −05 • 22 31 . 504 and the H 2 vibrational peak farther north, RA J2000 : 05 h 35 m 13 s .477, Dec J2000 : −05 • 22 08 .…”

“…After our detection of O 2 in Orion with Herschel (Goldsmith et al 2011;Chen et al 2014), and due to the difficulties of the interpretation, we decided to try to pinpoint the emission region by performing deep observations of the rarer 16 O 18 O isotopologue with ALMA at 234 GHz. To increase the interest of these observations, we took advantage of the possibility to divide the run into five independent observing blocks to move the backend windows as much as possible to cover the maximum bandwidth.…”