HD depletion in starless cores

Aims. We aim to investigate the abundances of light deuterium-bearing species such as HD, H2D+, and D2H+ in a gas-grain chemical model that includes an extensive description of deuterium and spin-state chemistry, in physical conditions appropriate to the very centers of starless cores. Methods. We c...

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Bibliographic Details
Published in:Astronomy and astrophysics (Berlin) 2013-06, Vol.554, p.1-14
Main Authors: Sipilä, O., Caselli, P., Harju, J.
Format: Article
Language:English
Subjects:
astrochemistry
Astronomy
Depletion
Deuterium
Dust
Evolution
Gas phases
ISM: abundances
ISM: clouds
ISM: molecules
Mathematical models
radiative transfer
Surface reactions
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Summary:Aims. We aim to investigate the abundances of light deuterium-bearing species such as HD, H2D+, and D2H+ in a gas-grain chemical model that includes an extensive description of deuterium and spin-state chemistry, in physical conditions appropriate to the very centers of starless cores. Methods. We combined a gas-grain chemical model with radiative transfer calculations to simulate density and temperature structure in starless cores. The chemical model includes new reaction sets for both gas phase and grain surface chemistry, including deuterated forms of species with up to four atoms and the spin states of the light species H2, H2+, and H2+ and their deuterated forms. Results. We find that in the dense and cold environments attributed to the centers of starless cores, HD eventually depletes from the gas phase because deuterium is efficiently incorporated into grain-surface HDO, resulting in inefficient HD production on grains for advanced core ages. HD depletion has consequences not only on the abundances of, e.g., H2D+ and D2H+, whose production depends on the abundance of HD, but also on the spin state abundance ratios of the various light species, when compared with the complete depletion model where heavy elements do not influence the chemistry. Conclusions. While the eventual HD depletion leads to the disappearance of light deuterium-bearing species from the gas phase on a relatively short timescale at high density, we find that at late stages of core evolution, the abundances of H2D+ and D2H+ increase toward the core edge, and the distributions become extended. The HD depletion timescale increases if less oxygen is initially present in the gas phase, owing to chemical interaction between the gas and the dust preceding the starless core phase. Our results are greatly affected if H2 is allowed to tunnel on grain surfaces, and therefore more experimental data is needed not only on tunneling but also on the O + H2 surface reaction in particular.
ISSN:0004-6361
1432-0746
DOI:10.1051/0004-6361/201220922