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

Deep searches for O₂ in emission and foreground absorption

¹ Stockholm Observatory, Stockholm University, AlbaNova University Center 106 91 Stockholm Sweden
e-mail: aage@astro.su.se
² Department of Earth and Space Sciences, Chalmers University of Technology, Onsala Space Observatory, 429 92 Onsala, Sweden
³ LERMA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, UPMC Univ. Paris 06, 75014 Paris, France
⁴ LERMA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, UPMC Univ. Paris 06, École Normale Supérieure, 75005 Paris, France
⁵ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena CA 91109, USA
⁶ California Institute of Technology, Cahill Center for Astronomy and Astrophysics 301-17, Pasadena, CA 91125, USA
⁷ LERMA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, UPMC Univ. Paris 06, 92190 Meudon, France
⁸ Department of Physics and Astronomy, University College London, London WC1E 6BT, UK

Received: 8 April 2015
Accepted: 18 October 2015

Abstract

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.