Herschel HIFI observations of the Sgr A +50 km s-1 Cloud

• Published in: Astronomy and astrophysics (Berlin) Vol. 584
• Main Authors: Sandqvist, Aa, Larsson, B., Hjalmarson, Å., Encrenaz, P., Gerin, M., Goldsmith, P. F., Lis, D. C., Liseau, R., Pagani, L., Roueff, E., Viti, S.
• Format: Journal Article
• Language: English
• Published: EDP Sciences 01.12.2015
• Subjects: Astrophysics; Galaxy: center; ISM: clouds; ISM: individual objects: Sgr A; ISM: molecules; Physics
• ISSN: 0004-6361; 1432-0746; 1432-0756
• DOI: 10.1051/0004-6361/201526280

Context. The Herschel Oxygen Project (HOP) is an open time key program, awarded 140 h of observing time to search for molecular oxygen (O2) in a number of interstellar sources. To date O2 has definitely been detected in only two sources, namely ρ Oph A and Orion, reflecting the extremely low abundance of O2 in the interstellar medium. Aims. One of the sources in the HOP program is the +50 km s-1 Cloud in the Sgr A Complex in the centre of the Milky Way. Its environment is unique in the Galaxy and this property is investigated to see if it is conducive to the presence of O2. Methods. The Herschel Heterodyne Instrument for the Far Infrared (HIFI) is used to search for the 487 and 774 GHz emission lines of O2. Results. No O2 emission is detected towards the Sgr A +50 km s-1 Cloud, but a number of strong emission lines of methanol (CH3OH) and absorption lines of chloronium (H2Cl+) are observed. Conclusions. A 3σ upper limit for the fractional abundance ratio of [O2]/[H2] in the Sgr A +50 km s-1 Cloud is found to be X(O2) ≤ 5 × 10-8. However, since we can find no other realistic molecular candidate than O2 itself, we very tentatively suggest that two weak absorption lines at 487.261 and 487.302 GHz may be caused by the 487 GHz line of O2 in two foreground spiral arm clouds. By considering that the absorption may only be apparent, the estimated upper limit to the O2 abundance of ≤ (10−20) × 10-6 in these foreground clouds is very high, as opposed to the upper limit in the Sgr A +50 km s-1 Cloud itself, but similar to what has been reached in recent chemical shock models for Orion. This abundance limit was determined also using Odin non-detection limits, and assumes that O2 fills the beam. If the absorption is due to a differential Herschel OFF-ON emission, the O2 fractional abundance may be of the order of ≈ (5−10) × 10-6. With the assumption of pure absorption by foreground clouds, the unreasonably high abundance of (1.4−2.8) × 10-4 was obtained. The rotation temperatures for CH3OH-A and CH3OH-E lines in the +50 km s-1 Cloud are found to be ≈ 64 and 79 K, respectively, and the fractional abundance of CH3OH is approximately 5 × 10-7.