Desorption of CO and O₂ interstellar ice analogs^*

¹ Raymond and Beverly Sackler Laboratory for Astrophysics, Leiden Observatory, Leiden University, Postbus 9513, 2300 RA Leiden, The Netherlands e-mail: acharyya@strw.leidenuniv.nl
² Centre For Space Physics, 43 Chalantika, Garia, Kolkata, 720084, India
³ Department of Physics SUPA (Scottish Universities Physics Alliance), University of Strathclyde, 107 Rottenrow East, Glasgow G4 ONG, Scotland

Received: 18 August 2006
Accepted: 30 January 2007

Abstract

Aims.Solid O₂ has been proposed as a possible reservoir for molecular oxygen in dense clouds through freeze-out processes. The aim of this work is to characterize quantitatively the physical processes that are involved in the desorption kinetics of CO–O₂ ices by interpreting laboratory temperature programmed desorption (TPD) data. This information is used to simulate the behavior of CO–O₂ ices under astrophysical conditions.

Methods.The TPD spectra have been recorded under ultra high vacuum conditions for pure, layered and mixed morphologies for different thicknesses, temperatures and mixing ratios. An empirical kinetic model is used to interpret the results and to provide input parameters for astrophysical models.

Results.Binding energies are determined for different ice morphologies. Independent of the ice morphology, the desorption of O₂ is found to follow 0th-order kinetics. Binding energies and temperature-dependent sticking probabilities for CO–CO, O₂–O₂ and CO–O₂ are determined. O₂ is slightly less volatile than CO, with a binding energy of versus  K for pure ices. In mixed and layered ices, CO does not co-desorb with O₂ but its binding energy is slightly increased compared to pure ice whereas that of O₂ is slightly decreased. Lower limits to the sticking probabilities of CO and O₂ are 0.9 and 0.85, respectively, at temperatures below 20 K. The balance between accretion and desorption is studied for O₂ and CO in astrophysically relevant scenarios. Only minor differences are found between the two species, i.e., both desorb between 16 and 18 K in typical environments around young stars. Thus, clouds with significant abundances of gaseous CO are unlikely to have large amounts of solid O₂.