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Simultaneous determination of the cosmic ray ionization rate and fractional ionization in DR 21(OH)

Hezareh, Talayeh and Houde, Martin and McCoey, Carolyn and Vastel, Charlotte and Peng, Ruisheng (2008) Simultaneous determination of the cosmic ray ionization rate and fractional ionization in DR 21(OH). Astrophysical Journal, 684 (2). pp. 1221-1227. ISSN 0004-637X.

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We present a new method for the simultaneous calculation of the cosmic ray ionization rate, ζH2, and the ionization fraction, χe, in dense molecular clouds. A simple network of chemical reactions dominant in the creation and destruction of HCNH^+ and HCO^+ is used in conjunction with observed pairs of rotational transitions of several molecular species in order to determine the electron abundance and the H3^+ abundance. The cosmic ray ionization rate is then calculated by taking advantage of the fact that, in dark clouds, it governs the rate of creation of H3^+. We apply this technique to the case of the star-forming region DR 21(OH), where we successfully detected the (J = 3 → 2) and (J = 4 → 3) rotational transitions of HCNH^+. We also determine the C and O isotopic ratios in this source to be ^12C/^13C = 63 ± 4 and ^16O/^18O = 318 ± 64, which are in good agreement with previous measurements in other clouds. The significance of our method lies in the ability to determine N(H3^+) and Xe directly from observations, and estimate ζH2 accordingly. Our results, ζH2 = 3.1 x 10^-18 s^-1 and χe = 3.2 x 10^-8, are consistent with recent determinations in other objects.

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Additional Information:© 2008 The American Astronomical Society. Received 2008 March 24; accepted 2008 May 20. The authors thank the anonymous referee for a careful reading and insightful comments, and also D. Johnstone and J. Di Francesco for helpful discussions and suggestions. M. H.’s research is funded through the NSERC Discovery Grant, Canada Research Chair, Canada Foundation for Innovation, Ontario Innovation Trust, and Western’s Academic Development Fund programs. The Caltech Submillimeter Observatory is funded through the NSF grant AST 05-40882 to the California Institute of Technology.
Funding AgencyGrant Number
NSFAST 05-40882
Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery GrantUNSPECIFIED
Canada Research ChairUNSPECIFIED
Canada Foundation for InnovationUNSPECIFIED
Ontario Innovation TrustUNSPECIFIED
Western’s Academic Development FundUNSPECIFIED
Subject Keywords:cosmic rays; ISM: clouds; ISM: molecules
Issue or Number:2
Record Number:CaltechAUTHORS:HEZapj08
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:13344
Deposited By: Tony Diaz
Deposited On:22 Apr 2009 20:55
Last Modified:03 Oct 2019 00:36

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