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Widespread Rotationally Hot Hydronium Ion in the Galactic Interstellar Medium

Lis, D. C. and Schilke, P. and Bergin, E. A. and Gerin, M. and Black, J. H. and Comito, C. and De Luca, M. and Godard, B. and Higgins, R. and Le Petit, F. and Pearson, J. C. and Pellegrini, E. W. and Phillips, T. G. and Yu, S. (2014) Widespread Rotationally Hot Hydronium Ion in the Galactic Interstellar Medium. Astrophysical Journal, 785 (2). Art. No. 135. ISSN 0004-637X. doi:10.1088/0004-637X/785/2/135.

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We present new Herschel observations of the (6,6) and (9,9) inversion transitions of the hydronium ion toward Sagittarius B2(N) and W31C. Sensitive observations toward Sagittarius B2(N) show that the high, ~500 K, rotational temperatures characterizing the population of the highly excited metastable H_3O^+ rotational levels are present over a wide range of velocities corresponding to the Sagittarius B2 envelope, as well as the foreground gas clouds between the Sun and the source. Observations of the same lines toward W31C, a line of sight that does not intersect the Central Molecular Zone but instead traces quiescent gas in the Galactic disk, also imply a high rotational temperature of ~380 K, well in excess of the kinetic temperature of the diffuse Galactic interstellar medium. While it is plausible that some fraction of the molecular gas may be heated to such high temperatures in the active environment of the Galactic center, characterized by high X-ray and cosmic-ray fluxes, shocks, and high degree of turbulence, this is unlikely in the largely quiescent environment of the Galactic disk clouds. We suggest instead that the highly excited states of the hydronium ion are populated mainly by exoergic chemical formation processes and the temperature describing the rotational level population does not represent the physical temperature of the medium. The same arguments may be applicable to other symmetric top rotors, such as ammonia. This offers a simple explanation of the long-standing puzzle of the presence of a pervasive, hot molecular gas component in the central region of the Milky Way. Moreover, our observations suggest that this is a universal process not limited to the active environments associated with galactic nuclei.

Item Type:Article
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URLURL TypeDescription Paper
Lis, D. C.0000-0002-0500-4700
Schilke, P.0000-0003-2141-5689
Bergin, E. A.0000-0003-4179-6394
Alternate Title:Widespread Rotationally-Hot Hydronium Ion in the Galactic Interstellar Medium
Additional Information:© 2014 American Astronomical Society. Received 2014 January 16; accepted 2014 March 4; published 2014 April 3. HIFI has been designed and built by a consortium of institutes and university departments from across Europe, Canada, and the United States (NASA) under the leadership of SRON, Netherlands Institute for Space Research, Groningen, The Netherlands, and with major contributions from Germany, France, and the US. Support for this work was provided by NASA (Herschel OT funding) through an award issued by JPL/ Caltech. The research of P.S., C.C., and R.H. is supported by the Collaborative Research Center 956 funded by the Deutsche Forschungsgemeinschaft (DFG).
Funding AgencyGrant Number
Deutsche Forschungsgemeinschaft (DFG)956
Subject Keywords:astrochemistry; galaxies: nuclei; ISM: molecules; molecular processes; submillimeter: general; techniques: spectroscopic
Issue or Number:2
Record Number:CaltechAUTHORS:20140401-111813634
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Official Citation:Widespread Rotationally Hot Hydronium Ion in the Galactic Interstellar Medium D. C. Lis et al. 2014 ApJ 785 135
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:44575
Deposited By: Ruth Sustaita
Deposited On:01 Apr 2014 20:26
Last Modified:10 Nov 2021 16:53

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