The influence of temperature on the synthesis of molecules on icy grain mantles in dense molecular clouds

¹ INAF - Osservatorio Astrofisico di Catania, via S. Sofia 78, 95123 Catania, Italy
mga@oact.inaf.it; mepalumbo@oact.inaf.it
² Astronomical Institute of Slovak Academy of Science, 059 60 Tatranska Lomnica, Slovakia
³ Raymond & Beverly Sackler Laboratory for Astrophysics, Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands

Received: 6 July 2010
Accepted: 2 February 2011

Abstract

Context. Infrared observations show the presence of icy mantles along the line of sight toward young stellar objects (YSOs), where a temperature gradient is expected and indirectly observed. In this environment, icy mantles are affected by ion and UV irradiation. Laboratory experiments show that molecules are formed after irradiation of icy mixtures. However, most of the experiments done so far have been performed in the temperatures range of 10–20 K.

Aims. To extend previous work we irradiated some icy mixtures, namely H₂O:CO=10:1, H₂O:CH₄=4:1, and H₂O:CO₂=3:1 at two different temperatures (12 K and 40 or 60 K) to study the effects of temperature on the synthesis of molecules and the decrease in their parent species after ion irradiation.

Methods. The experiments were performed in a high-vacuum chamber (P < 10^-7 mbar), where icy samples were irradiated with 30 keV He⁺ ions and analyzed by a FTIR spectrophotometer. Infrared spectra of the samples were recorded after various steps of irradiation.

Results. We found that the temperature affects the behavior of the volatile species (i.e., CO and CH₄) during irradiation. As a consequence, the production of molecular species is generally more prevalent at 12 K than at either 40 or 60 K, while the decrease in their parent volatile species is faster at high temperature.

Conclusions. We conclude that the behavior of each species depends on the value of its sublimation temperature with respect to the temperature of the sample. If this latter is higher than the sublimation temperature of a given species, then the effects of thermal desorption compete with those due to irradiation.