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Gas-Grain Models for Interstellar Anion ChemistryLong-chain hydrocarbon anions C(sub n) H(-) (n = 4, 6, 8) have recently been found to be abundant in a variety of interstellar clouds. In order to explain their large abundances in the denser (prestellar/protostellar) environments, new chemical models are constructed that include gas-grain interactions. Models including accretion of gas-phase species onto dust grains and cosmic-ray-induced desorption of atoms are able to reproduce the observed anion-to-neutral ratios, as well as the absolute abundances of anionic and neutral carbon chains, with a reasonable degree of accuracy. Due to their destructive effects, the depletion of oxygen atoms onto dust results in substantially greater polyyne and anion abundances in high-density gas (with n(sub H2) approx > / cubic cm). The large abundances of carbon-chain-bearing species observed in the envelopes of protostars such as L1527 can thus be explained without the need for warm carbon-chain chemistry. The C6H(-) anion-to-neutral ratio is found to be most sensitive to the atomic O and H abundances and the electron density. Therefore, as a core evolves, falling atomic abundances and rising electron densities are found to result in increasing anion-to-neutral ratios. Inclusion of cosmic-ray desorption of atoms in high-density models delays freeze-out, which results in a more temporally stable anion-to-neutral ratio, in better agreement with observations. Our models include reactions between oxygen atoms and carbon-chain anions to produce carbon-chain-oxide species C6O, C7O, HC6O, and HC7O, the abundances of which depend on the assumed branching ratios for associative electron detachment
Document ID
20120010132
Document Type
Reprint (Version printed in journal)
Authors
Cordiner, M. A. (Catholic Univ. of America Washington, DC, United States)
Charnely, S. B. (NASA Goddard Space Flight Center Greenbelt, MD, United States)
Date Acquired
August 25, 2013
Publication Date
April 20, 2012
Publication Information
Publication: The Astrophysical Journal
Volume: 749
Issue: 2
Subject Category
Astrophysics
Report/Patent Number
GSFC.JA.00166.2012
Distribution Limits
Public
Copyright
Other