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Hot, metastable hydronium ion in the Galactic centre: formation pumping in X-ray-irradiated gas?

Lis, Dariusz C. and Schilke, Peter and Bergin, Edwin A. and Emprechtinger, Martin (2012) Hot, metastable hydronium ion in the Galactic centre: formation pumping in X-ray-irradiated gas? Philosophical Transactions A: Mathematical, Physical and Engineering Sciences, 370 (1978). pp. 5162-5173. ISSN 1364-503X. doi:10.1098/rsta.2012.0025.

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With a 3.5 m diameter telescope passively cooled to approximately 80 K, and a science payload comprising two direct detection cameras/medium resolution imaging spectrometers (PACS and SPIRE) and a very high spectral resolution heterodyne spectrometer (HIFI), the Herschel Space Observatory is providing extraordinary observational opportunities in the 55–670 μm spectral range. HIFI has opened for the first time to high-resolution spectroscopy the submillimetre band that includes the fundamental rotational transitions of interstellar hydrides, the basic building blocks of astrochemistry. We discuss a recent HIFI discovery of metastable rotational transitions of the hydronium ion (protonated water, H_(3)O^+), with rotational level energies up to 1200 K above the ground state, in absorption towards Sagittarius B2(N) in the Galactic centre. Hydronium is an important molecular ion in the oxygen chemical network. Earlier HIFI observations have indicated a general deficiency of H_(3)O^+ in the diffuse gas in the Galactic disc. The presence of hot H_(3)O^+ towards Sagittarius B2(N) thus appears to be related to the unique physical conditions in the central molecular zone, manifested, for example, by the widespread presence of abundant H^(+)_(3). One intriguing theory for the high rotational temperature characterizing the population of the H3O+ metastable levels may be formation pumping in molecular gas irradiated by X-rays emitted by the Galactic centre black hole. Alternatively, the pervasive presence of enhanced turbulence in the central molecular zone may give rise to shocks in the lower-density medium that is exposed to energetic radiation.

Item Type:Article
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URLURL TypeDescription
Lis, Dariusz C.0000-0002-0500-4700
Schilke, Peter0000-0003-2141-5689
Bergin, Edwin A.0000-0003-4179-6394
Additional Information:© 2012 The Royal Society. We thank E. Roueff, T. Oka and an anonymous referee for helpful comments. HIFI has been designed and built by a consortium of institutes and university departments from across Europe, Canada and the United States under the leadership of SRON Netherlands Institute for Space Research, Groningen, The Netherlands and with major contributions from Germany, France and the USA. Consortium members are: Canada: CSA, University of Waterloo; France: CESR, LAB, LERMA, IRAM; Germany: KOSMA, MPIfR, MPS; Ireland: NUI Maynooth; Italy: ASI, IFSI– INAF, Osservatorio Astrofisico di Arcetri–INAF; Netherlands: SRON, TUD; Poland: CAMK, CBK; Spain: Observatorio Astronmico Nacional (IGN), Centro de Astrobiologa (CSIC-INTA); Sweden: Chalmers University of Technology–MC2, RSS & GARD, Onsala Space Observatory, Swedish National Space Board, Stockholm University–Stockholm Observatory; Switzerland: ETH Zurich, FHNW; USA: Caltech, JPL, NHSC. Support for this work was provided by NASA through an award issued by JPL/Caltech.
Funding AgencyGrant Number
Subject Keywords:astrochemistry; interstellar medium: abundances; interstellar medium: molecules; molecular processes; submillimetre: interstellar medium
Issue or Number:1978
Record Number:CaltechAUTHORS:20121113-095754362
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:35421
Deposited By: Jason Perez
Deposited On:13 Nov 2012 19:03
Last Modified:09 Nov 2021 23:15

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