Gas-grain models for interstellar anion chemistry

Abstract

Long-chain hydrocarbon anions CnH- (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_{H_2} >~ 10^5 cm^{-3}). 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.

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Cited by in corpus (13)

• The UMIST database for astrochemistry 2012
• Discovery of HC3O+ in space: The chemistry of O-bearing species in TMC-1
• Detection of Interstellar HC${}_5$O in TMC-1 with the Green Bank Telescope
• On the ubiquity of molecular anions in the dense interstellar medium
• Deep K-band observations of TMC-1 with the Green Bank Telescope: Detection of HC7O, non-detection of HC11N, and a search for new organic molecules
• Discovery of HCCCO and C5O in TMC-1 with the QUIJOTE line survey
• The abundance and excitation of molecular anions in interstellar clouds
• Evidence for two-electron processes in the mutual neutralization of O- with O+ and N+ at Subthermal Collision Energies
• A Search for O_2 in CO-depleted Molecular Cloud Cores with Herschel
• The connection between warm carbon chain chemistry and interstellar irradiation of star-forming cores
• Modeling the role of electron attachment rates on column density ratios for CnH-/CnH (n=4,6,8) in dense molecular clouds
• New Bistable Solutions in Molecular Cloud Chemistry: Nitrogen and Carbon Autocatalysis
• High-level ab initio quartic force fields and spectroscopic characterization of C${}_2$N${}^{-}$