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The Fractional Ionization in Molecular Cloud Cores

Published online by Cambridge University Press:  25 May 2016

Paola Caselli
Paola Caselli*
Affiliation:
Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, I-50125 Firenze, Italy

Abstract

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Ions and electrons play a key role in the chemical and dynamical evolution of interstellar clouds. Gas phase ion–molecule reactions are major chemical routes to the formation of interstellar molecules. The ionization degree determines the coupling between the magnetic field and the molecular gas through ion–neutral collisions, and thus regulates the rate of star formation. In the theoretical determination of the degree of ionization we run into several sources of uncertainty, including the poorly known cosmic ray flux and metal depletion within the cores, the penetration of UV radiation deep into regions of high visual extinction due to cloud inhomogeneities, and the ionization rate increase in the proximity of young stellar objects which may be strong X–ray emitters. Observational estimates of electron (or ion) fractions x(e) (≡ n(e)/n(H2), where n(e) and n(H2) are the electron and molecular hydrogen number densities, respectively) in dense cloud cores are thus of considerable interest. In this paper, I will review recent improvements in the estimates of the ion fraction in dense cores and point out the difficulties in determining x(e).

Type
Part 1. Chemistry in Pre-Stellar Cores and Low-Mass Star-Forming Regions
Information
Symposium - International Astronomical Union , Volume 197: Astrochemistry: From Molecular Clouds to Planetary Systems , 2000 , pp. 41 - 50
Copyright
Copyright © Astronomical Society of the Pacific 2000 

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