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

Abstract

Aims. We aim to investigate whether a multilayer ice model can be as successful as a bulk ice model in reproducing the observed abundances of various deuterated gas-phase species toward starless cores. Methods. We calculate abundances for various deuterated species as functions of time adopting fixed physical conditions. We also estimate abundance gradients 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 $\rm DCO^+$ and $\rm N_2D^+$ that are about an order of magnitude lower than observed, caused by the trapping of CO and $\rm N_2$ into 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 what has been observed. Another distinguishing feature of the multilayer model is that $\rm D_3^+$ 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 be better in reproducing observations than the multilayer ice model. According to our results, the $\rm H_2D^+$ to $\rm N_2D^+$ abundance ratio is higher than 100 in the multilayer model, while only a few $\times$ 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 $\rm D_3^+$ compared to $\rm H_2D^+$ and $\rm D_2H^+$ would provide additional constraints for the models.