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J. Scalo (Univ. Texas at Austin), P. Padoan (Univ.Calif. San Diego)
Motivated by the recent determination of a large value of the cosmic ray ionization rate in the diffuse line of sight toward \zeta Persei by McCall et al. (2003), we propose a mechanism for producing variations in the interstellar ionization rate. In neutral or molecular ISM regions, cosmic ray self-confinement due to resonant scattering off self-generated magnetic waves determines the cosmic ray streaming velocity. Flux conservation then leads to variations in the cosmic ray number density, which we show are a function of gas density and ionization fraction. Based on this mechanism, we find that the value of the cosmic ray ionization rate in diffuse gas at a density of the order of 100 cm-3 may be up to 50 times larger than in dense molecular cloud cores, primarily because of the large decrease in ionization fraction in regions shielded from UV radiation. At lower densities in diffuse regions we predict that the rate is proportional to the square root of the gas density. Variations of the rate within molecular cloud cores are predicted to be small, with the rate becoming constant at very large densities, where self-confinement is not effective. Such variations of the cosmic ray ionization rate in diffuse and dark interstellar gas appear to be consistent with the observational results. We illustrate the variation by applying our solutions for the cosmic ray ionization rate to simulations of interstellar density structure.
J.S. acknowledges support from NASA ATP grant NAG5-13280 and Exobiology grant NNG04GK43G.
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Bulletin of the American Astronomical Society, 36 5
© 2004. The American Astronomical Society.