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Ubiquitous argonium (ArH^+) in the diffuse interstellar medium: A molecular tracer of almost purely atomic gas

Schilke, P. and Neufeld, D. A. and Müller, H. S. P. and Comito, C. and Bergin, E. A. and Lis, D. C. and Gerin, M. and Black, J. H. and Wolfire, M. and Indriolo, N. and Pearson, J. C. and Menten, K. M. and Winkel, B. and Sánchez-Monge, Á. and Möller, T. and Godard, B. and Falgarone, E. (2014) Ubiquitous argonium (ArH^+) in the diffuse interstellar medium: A molecular tracer of almost purely atomic gas. Astronomy and Astrophysics, 566 . Art. No. A29. ISSN 0004-6361.

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Aims. We describe the assignment of a previously unidentified interstellar absorption line to ArH^+ and discuss its relevance in the context of hydride absorption in diffuse gas with a low H_2 fraction. The confidence of the assignment to ArH^+ is discussed, and the column densities are determined toward several lines of sight. The results are then discussed in the framework of chemical models, with the aim of explaining the observed column densities. Methods. We fitted the spectral lines with multiple velocity components, and determined column densities from the line-to-continuum ratio. The column densities of ArH^+ were compared to those of other species, tracing interstellar medium (ISM) components with different H_2 abundances. We constructed chemical models that take UV radiation and cosmic ray ionization into account. Results. Thanks to the detection of two isotopologues, ^(36)ArH^+ and ^(38)ArH^+, we are confident about the carrier assignment to ArH^+. NeH^+ is not detected with a limit of [NeH^+]/[ArH^+] ≤ 0.1. The derived column densities agree well with the predictions of chemical models. ArH^+ is a unique tracer of gas with a fractional H_2 abundance of 10^(-4) − 10^(-3) and shows little correlation to H_2O^+, which traces gas with a fractional H_2 abundance of ≈0.1. Conclusions. A careful analysis of variations in the ArH^+, OH^+, H_2O^+, and HF column densities promises to be a faithful tracer of the distribution of the H_2 fractional abundance by providing unique information on a poorly known phase in the cycle of interstellar matter and on its transition from atomic diffuse gas to dense molecular gas traced by CO emission. Abundances of these species put strong observational constraints upon magnetohydrodynamical (MHD)simulations of the interstellar medium, and potentially could evolve into a tool characterizing the ISM. Paradoxically, the ArH^+ molecule is a better tracer of almost purely atomic hydrogen gas than Hi itself, since Hi can also be present in gas with a significant molecular content, but ArH^+ singles out gas that is >99.9% atomic.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Schilke, P.0000-0003-2141-5689
Neufeld, D. A.0000-0001-8341-1646
Müller, H. S. P.0000-0002-0183-8927
Bergin, E. A.0000-0003-4179-6394
Lis, D. C.0000-0002-0500-4700
Wolfire, M.0000-0003-0030-9510
Indriolo, N.0000-0001-8533-6440
Menten, K. M.0000-0001-6459-0669
Alternate Title:Ubiquitous argonium (ArH+) in the diffuse interstellar medium: A molecular tracer of almost purely atomic gas
Additional Information:© 2014 ESO. Received 28 February 2014. Accepted 29 March 2014. Published online 04 June 2014. 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 US. Consortium members are: Canada: CSA, U. 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 Astronomico Nacional (IGN), Centro de Astrobiologia (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. H.S.P.M. is very grateful to the Bundesministerium für Bildung und Forschung (BMBF) for initial support through project FKZ 50OF0901 (ICC HIFI Herschel) aimed at maintaining the Cologne Database for Molecular Spectroscopy, CDMS. This support has been administered by the Deutsches Zentrum für Luft- und Raumfahrt (DLR). Part of this work was supported by the German Deutsche Forschungsgemeinschaft in the Collaborative Research Center SFB956, and by the German Ministry of Science (BMBF) trough contract 05A11PK3. This work also has been supported by NASA through an award issued by JPL/Caltech. We thank Christian Endres for tireless work on the molecular line catalog implementation.
Funding AgencyGrant Number
Bundesministerium für Bildung und Forschung (BMBF)FKZ 50OF0901
Deutsches Zentrum für Luft- und Raumfahrt (DLR)UNSPECIFIED
Deutsche Forschungsgemeinschaft (DFG)SFB956
Bundesministerium für Bildung und Forschung (BMBF)05A11PK3
Subject Keywords:astrochemistry, line: identification, molecular processes, ISM: abundances, ISM: molecules, ISM: structure
Record Number:CaltechAUTHORS:20140808-133328585
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Official Citation:Ubiquitous argonium (ArH+) in the diffuse interstellar medium: A molecular tracer of almost purely atomic gas P. Schilke, D. A. Neufeld, H. S. P. Müller, C. Comito, E. A. Bergin, D. C. Lis, M. Gerin, J. H. Black, M. Wolfire, N. Indriolo, J. C. Pearson, K. M. Menten, B. Winkel, Á. Sánchez-Monge, T. Möller, B. Godard and E. Falgarone A&A 566 A29 (2014) DOI:
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:48249
Deposited By: Ruth Sustaita
Deposited On:08 Aug 2014 21:38
Last Modified:09 Mar 2020 13:19

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