Herschel HIFI observations of the Sgr A+50 km s(-1) Cloud Deep searches for O-2 in emission and foreground absorption
Artikel i vetenskaplig tidskrift, 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


A. Sandqvist

Stockholms observatorium

B. Larsson

Stockholms observatorium

Åke Hjalmarson

Chalmers, Rymd- och geovetenskap, Radioastronomi och astrofysik

P. Encrenaz

Sorbonne Université

M. Gerin

Sorbonne Université

P. F. Goldsmith

Jet Propulsion Laboratory, California Institute of Technology

D. C. Lis

Sorbonne Université

California Institute of Technology (Caltech)

René Liseau

Chalmers, Rymd- och geovetenskap, Radioastronomi och astrofysik

L. Pagani

Sorbonne Université

E. Roueff

LERMA - Laboratoire d'Etudes du Rayonnement et de la Matiere en Astrophysique et Atmospheres

S. Viti

University College London (UCL)

Astronomy and Astrophysics

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

Vol. 584 A118


Astronomi, astrofysik och kosmologi


Onsala rymdobservatorium



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