2015A&A...584A.118S

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. One of the sources in the HOP program is the +50km/s 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. The Herschel Heterodyne Instrument for the Far Infrared (HIFI) is used to search for the 487 and 774 GHz emission lines of O₂. No O₂ emission is detected towards the Sgr A +50km/s Cloud, but a number of strong emission lines of methanol (CH₃OH) and absorption lines of chloronium (H₂Cl⁺) are observed. A 3σ upper limit for the fractional abundance ratio of [O₂]/[H₂] in the Sgr A +50km/s Cloud is found to be X(O₂)≤5x10^–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)x10^–6 in these foreground clouds is very high, as opposed to the upper limit in the Sgr A +50km/s 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)x10^–6. With the assumption of pure absorption by foreground clouds, the unreasonably high abundance of (1.4-2.8)x10^–4 was obtained. The rotation temperatures for CH₃OH-A and CH₃OH-E lines in the +50km/s Cloud are found to be ≃64 and 79K, respectively, and the fractional abundance of CH₃OH is approximately 5x10^–7.