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

Sandqvist, Aa. and Larsson, B. and Hjalmarson, Å. and Encrenaz, P. and Gerin, M. and Goldsmith, P. F. and Lis, D. C. and Liseau, R. and Pagani, L. and Roueff, E. and Viti, S. (2015) Herschel HIFI Observations of the Sgr A +50 km s^(-1) Cloud. Deep Searches for O_2 in Emission and Foreground Absorption. Astronomy and Astrophysics, 584 . Art. No. A118. ISSN 0004-6361.

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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).

Item Type:Article
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URLURL TypeDescription Paper
Goldsmith, P. F.0000-0002-6622-8396
Lis, D. C.0000-0002-0500-4700
Roueff, E.0000-0002-4949-8562
Alternate Title:Herschel HIFI observations of the Sgr A +50 km s-1 Cloud. Deep searches for O2 in emission and foreground absorption
Additional Information:© ESO, 2015. Received 8 April 2015 / Accepted 18 October 2015. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA. We wish to thank Per Olof Lindblad for useful discussions and comments concerning Galactic density waves and resulting shock interaction with Galactic spiral arms. We are very grateful to our anonymous referee whose thorough report significantly improved this paper. We also wish to thank the Swedish National Space Board (SNSB) for its continued financial support. Furthermore we express our appreciation to the individuals making, updating and maintaining the Splatalogue, CDMS and JPL molecular spectroscopy data bases for their unselfish demanding work. HIFI has been designed and built by a consortium of institutes and university departments from across Europe, Canada and the US under the leadership of SRON Netherlands Institute for Space Research, Groningen, The Netherlands with major contributions from Germany, France and the US. Consortium members are: Canada: CSA, U. Waterloo; France: CESR, LAB, LERMA, IRAM; Germany: KOSMA, MPIfR, MPS; Ireland, NUI Maynooth; It aly: ASI, IFSI-INAF, Arcetri-INAF; Netherlands: SRON, TUD; Poland: CAMK, CBK; Spain: Observatorio Astronomico Nacional (IGN), Centro de Astrobiologia (CSIC-INTA); Sweden: Chalmers University of Technology MC2, RSS and GARD, Onsala Space Observatory, Swedish National Space Board, Stockholm University Stockholm Observatory; Switzerland: ETH Zürich, FHNW; USA: Caltech, JPL, NHSC. Support for this work was provided by NASA through an award issued by JPL/Caltech.
Funding AgencyGrant Number
Swedish National Space Board (SNSB)UNSPECIFIED
Subject Keywords:Galaxy: center / ISM: individual objects: Sgr A / ISM: molecules / ISM: clouds
Record Number:CaltechAUTHORS:20151118-074932833
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Official Citation:Herschel HIFI observations of the Sgr A +50 km s-1 Cloud - Deep searches for O2 in emission and foreground absorption Aa. Sandqvist, B. Larsson, Å. Hjalmarson, P. Encrenaz, M. Gerin, P. F. Goldsmith, D. C. Lis, R. Liseau, L. Pagani, E. RoueffAa. Sandqvist, B. Larsson, Å. Hjalmarson, P. Encrenaz, M. Gerin, P. F. Goldsmith, D. C. Lis, R. Liseau, L. Pagani, E. Roueff and S. Viti A&A, 584 (2015) A118 DOI:
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:62185
Deposited By: Tony Diaz
Deposited On:18 Nov 2015 19:27
Last Modified:09 Mar 2020 13:19

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