Letter to the Editor

Cold gas as an ice diagnostic toward low mass protostars^*

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands e-mail: oberg@strw.leidenuniv.nl
² Max-Planck-Institut für extraterrestrische Physik (MPE), Giessenbachstraat 1, 85748 Garching, Germany

Received: 27 October 2008
Accepted: 22 December 2008

Abstract

Context. Up to 90% of the chemical reactions during star formation occurs on ice surfaces, probably including the formation of complex organics. Only the most abundant ice species are however observed directly by infrared spectroscopy.

Aims. This study aims to develop an indirect observational method of ices based on non-thermal ice desorption in the colder part of protostellar envelopes.

Methods. The IRAM 30 m telescope was employed to observe two molecules that can be detected both in the gas and the ice, CH₃OH and HNCO, toward 4 low mass embedded protostars. Their respective gas-phase column densities are determined using rotational diagrams. The relationship between ice and gas phase abundances is subsequently determined.

Results. The observed gas and ice abundances span several orders of magnitude. Most of the CH₃OH and HNCO gas along the lines of sight is inferred to be quiescent from the measured line widths and the derived excitation temperatures, and hence not affected by thermal desorption close to the protostar or in outflow shocks. The measured gas to ice ratio of ~10^-4 agrees well with model predictions for non-thermal desorption under cold envelope conditions and there is a tentative correlation between ice and gas phase abundances. This indicates that non-thermal desorption products can serve as a signature of the ice composition. A larger sample is however necessary to provide a conclusive proof of concept.