Herschel HIFI Observations of the Sgr A +50 km s^(-1) Cloud. Deep Searches for O_2 in Emission and Foreground Absorption

Abstract

Context. The Herschel Oxygen Project (HOP) is an open time key program, awarded 140 h of observing time to search for molecular oxygen (O_2) in a number of interstellar sources. To date O_2 has definitely been detected in only two sources, namely ρ Oph A and Orion, reflecting the extremely low abundance of O_2 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 O_2. Methods. The Herschel Heterodyne Instrument for the Far Infrared (HIFI) is used to search for the 487 and 774 GHz emission lines of O_2. Results. No O_2 emission is detected towards the Sgr A +50 km s^(-1) Cloud, but a number of strong emission lines of methanol (CH_3OH) and absorption lines of chloronium (H_2Cl^+) are observed. Conclusions. A 3σ upper limit for the fractional abundance ratio of [O_2]/[H_2] in the Sgr A +50 km s^(-1) Cloud is found to be X(O_2) ≤ 5 × 10^(-8). However, since we can find no other realistic molecular candidate than O_2 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 O_2 in two foreground spiral arm clouds. By considering that the absorption may only be apparent, the estimated upper limit to the O_2 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 O_2 fills the beam. If the absorption is due to a differential Herschel OFF-ON emission, the O_2 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 CH_3OH-A and CH_3OH-E lines in the +50 km s^(-1) Cloud are found to be ≈ 64 and 79 K, respectively, and the fractional abundance of CH_3OH is approximately 5 × 10^(-7).