NON-EQUILIBRIUM CHEMISTRY OF DYNAMICALLY EVOLVING PRESTELLAR CORES. II. IONIZATION AND MAGNETIC FIELD

We study the effect that non-equilibrium chemistry in dynamical models of collapsing molecular cloud cores has on measurements of the magnetic field in these cores, the degree of ionization, and the mean molecular weight of ions. We find that OH and CN, usually used in Zeeman observations of the lin...

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Bibliographic Details
Published in:The Astrophysical journal 2012-07, Vol.754 (1), p.1-8
Main Authors: Tassis, Konstantinos, Willacy, Karen, Yorke, Harold W, Turner, Neal J
Format: Article
Language:English
Subjects:
AMBIPOLAR DIFFUSION
ASTRONOMY
ASTROPHYSICS
ASTROPHYSICS, COSMOLOGY AND ASTRONOMY
Asymptotic properties
CARBON NITRIDES
COSMIC RADIATION
COSMOCHEMISTRY
DENSITY
Dominance
EQUILIBRIUM
HYDROGEN IONS 1 PLUS
IONIZATION
LAYERS
Line of sight
MAGNETIC CORES
MAGNETIC FIELDS
MAGNETOHYDRODYNAMICS
Molecular weight
MOLECULES
STARS
ZEEMAN EFFECT
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Summary:We study the effect that non-equilibrium chemistry in dynamical models of collapsing molecular cloud cores has on measurements of the magnetic field in these cores, the degree of ionization, and the mean molecular weight of ions. We find that OH and CN, usually used in Zeeman observations of the line-of-sight magnetic field, have an abundance that decreases toward the center of the core much faster than the density increases. As a result, Zeeman observations tend to sample the outer layers of the core and consistently underestimate the core magnetic field. The degree of ionization follows a complicated dependence on the number density at central densities up to 10 super(5) cm super(-3) for magnetic models and 10 super(6) cm super(-3) in non-magnetic models. At higher central densities, the scaling approaches a power law with a slope of -0.6 and a normalization which depends on the cosmic-ray ionization rate [zeta] and the temperature T as ([zeta]T) super(1/2). The mean molecular weight of ions is systematically lower than the usually assumed value of 20-30, and, at high densities, approaches a value of 3 due to the asymptotic dominance of the H super(+) sub(3) ion. This significantly lower value implies that ambipolar diffusion operates faster.
ISSN:0004-637X
1538-4357
DOI:10.1088/0004-637X/754/1/6