On the enhanced cosmic-ray ionization rate in the diffuse cloud toward ζ persei

Shaw, Gargi ; Ferland, G. J. ; Srianand, R. ; Abel, N. P. ; van Hoof, P. A. M. ; Stancil, P. C. (2008) On the enhanced cosmic-ray ionization rate in the diffuse cloud toward ζ persei The Astrophysical Journal, 675 (1). pp. 405-412. ISSN 0004-637X

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Official URL: http://iopscience.iop.org/0004-637X/675/1/405

Related URL: http://dx.doi.org/10.1086/526395

Abstract

The spatial distribution of the cosmic-ray flux is important in understanding the interstellar medium (ISM) of the Galaxy. This distribution can be analyzed by studying different molecular species along different sight lines whose abundances are sensitive to the cosmic-ray ionization rate. Recently several groups have reported an enhanced cosmic-ray ionization rate (ζ = χCRζstandard) in diffuse clouds compared to the standard value, ζstandard (=2.5 × 10-17 s-1), measured toward dense molecular clouds. In an earlier work we reported an enhancement χCR = 20 toward HD 185418. McCall et al. have reported χCR = 48 toward ζ Persei based on the observed abundance of H+3, while Le Petit et al. found χCR ≈ 10 to be consistent with their models for this same sight line. Here we revisit ζ Persei and perform a detailed calculation using a self-consistent treatment of the hydrogen chemistry, grain physics, energy and ionization balance, and excitation physics. We show that the value of χCR deduced from the H+3 column density, N(H+3) , in the diffuse region of the sight line depends strongly on the properties of the grains because they remove free electrons and change the hydrogen chemistry. The observations are largely consistent with χCR ≈ 40, with several diagnostics indicating higher values. This underscores the importance of a full treatment of grain physics in studies of interstellar chemistry.

Item Type:Article
Source:Copyright of this article belongs to University of Chicago Press.
ID Code:55236
Deposited On:18 Aug 2011 08:10
Last Modified:18 May 2016 07:35

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