Effect of multilayer ice chemistry on gas-phase deuteration in starless cores

¹ Max-Planck-Institute for Extraterrestrial Physics (MPE), Giessenbachstr. 1, 85748 Garching, Germany
e-mail: osipila@mpe.mpg.de
² Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands

Received: 8 February 2016
Accepted: 15 April 2016

Abstract

Context. Astrochemical models commonly used to study the deuterium chemistry in starless cores consider a two-phase approach in which the ice on the dust grains is assumed to be entirely reactive. Recent experimental studies suggest that cold interstellar ices are mostly inert, and a multilayer model distinguishing the chemical processes at the surface and in the ice bulk would be more appropriate.

Aims. We investigate whether the multilayer model can be as successful as the bulk model in reproducing the observed abundances of various deuterated gas-phase species toward starless cores.

Methods. We calculated abundances for various deuterated species as functions of time using a pseudo-time-dependent chemical model adopting fixed physical conditions. We also estimated abundance gradients in starless cores by adopting a modified Bonnor-Ebert sphere as a core model. In the multilayer ice scenario, we consider desorption from one or several monolayers on the surface.

Results. We find that the multilayer model predicts abundances of DCO⁺ and N₂D⁺ that are about an order of magnitude lower than observed; the difference is caused by the trapping of CO and N₂ within the grain mantle. As a result of the mantle trapping, deuteration efficiency in the gas phase increases and we find stronger deuterium fractionation in ammonia than has been observed. Another distinguishing feature of the multilayer model is that D₃⁺ becomes the main deuterated ion at high density. The bulk ice model is generally easily reconciled with observations.

Conclusions. Our results underline that more theoretical and experimental work is needed to understand the composition and morphology of interstellar ices, and the desorption processes that can act on them. With the current constraints, the bulk ice model appears to reproduce the observations more accurately than the multilayer ice model. According to our results, the abundance ratio of H₂D⁺ to N₂D⁺ is higher than 100 in the multilayer model, while only a few ×10 in the bulk model, and so observations of this ratio could provide information on the ice morphology in starless cores. Observations of the abundance of D₃⁺ compared to H₂D⁺ and D₂H⁺, although challenging, would provide additional constraints for the models.