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

Författare

A. Sandqvist

Stockholm Observatory

B. Larsson

Stockholm Observatory

Åke Hjalmarson

Chalmers, Rymd- och geovetenskap, Radioastronomi och astrofysik

P. Encrenaz

Universite Paris Sorbonne - Paris IV

M. Gerin

Universite Paris Sorbonne - Paris IV

P. F. Goldsmith

Jet Propulsion Laboratory, California Institute of Technology

D. C. Lis

Universite Paris Sorbonne - Paris IV

CalTech

René Liseau

Chalmers, Rymd- och geovetenskap, Radioastronomi och astrofysik

L. Pagani

Universite Paris Sorbonne - Paris IV

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

Ämneskategorier

Astronomi, astrofysik och kosmologi

Infrastruktur

Onsala rymdobservatorium

DOI

10.1051/0004-6361/201526280