Herschel HIFI observations of the Sgr A+50 km s(-1) Cloud Deep searches for O-2 in emission and foreground absorption
Journal article, 2015

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 rho 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 (CH3OH) and absorption lines of chloronium (H2Cl+) are observed. Conclusions. A 3 sigma 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) <= 5x 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) x 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 approximate to(5-10) x 10 (6). With the assumption of pure absorption by foreground clouds, the unreasonably high abundance of (1.4-2.8) x 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 approximate to 64 and 79 K, respectively, and the fractional abundance of CH3OH is approximately 5 x 10(-7).

Galaxy: center

ISM: molecules

ISM: clouds

ISM: individual objects: Sgr A

Author

[Person 49d8f972-afb7-47fb-ad2b-138bcdbfe0d9 not found]

[Person 94d5b15b-782e-4790-b31d-312a6b6fc8ba not found]

[Person fccce444-137e-49f8-99ef-f972171a5bf9 not found]

[Person a7c6c961-3917-4c0f-9ea6-72e8fdcdfce3 not found]

[Person 1d5c5824-fc21-4128-a77b-47a839c7f19d not found]

[Person da72e8c5-8d46-461f-83f6-24ff5f746b64 not found]

[Person e5ec2f5a-e65f-4642-bff2-555fbc776d9f not found]

[Person 7191f197-5af1-4d46-8ac6-fab9f54c2238 not found]

[Person 317494ad-16a8-4d7c-b186-8f99c2b1edae not found]

[Person c2b4b890-5156-41e8-8ed1-f5f2b37e7623 not found]

[Person 573f8ef2-bb31-4a3b-901d-ed25fd640b5d not found]

Astronomy and Astrophysics

0004-6361 (ISSN) 1432-0746 (eISSN)

Vol. 584

Subject Categories

Astronomy, Astrophysics and Cosmology

Infrastructure

Onsala Space Observatory

DOI

10.1051/0004-6361/201526280

More information

Latest update

4/13/2018