Desorption rates and sticking coefficients for CO and N₂ interstellar ices

¹ Raymond and Beverly Sackler Laboratory for Astrophysics at Leiden Observatory, Postbus 9513, 2300 RA Leiden, The Netherlands e-mail: bisschop@strw.leidenuniv.nl
² Department of Physics, University of Strathclyde, 107 Rottenrow East, Glasgow G4 ONG, Scotland
³ Division of Geological and Planetary Sciences, California Institute of Technology, Mail Stop 150-21, Pasadena, CA 91125, USA
⁴ I. Physikalisches Institut, Universität zu Köln, Zulpicher Strasse 77, 50937 Köln, Germany

Received: 15 August 2005
Accepted: 30 December 2005

Abstract

We present Temperature Programmed Desorption (TPD) experiments of CO and N₂ ices in pure, layered and mixed morphologies at various ice “thicknesses” and abundance ratios as well as simultaneously taken Reflection Absorption Infrared Spectra (RAIRS) of CO. A kinetic model has been developed to constrain the binding energies of CO and N₂ in both pure and mixed environments and to derive the kinetics for desorption, mixing and segregation. For mixed ices N₂ desorption occurs in a single step whereas for layered ices it proceeds in two steps, one corresponding to N₂ desorption from a pure N₂ ice environment and one corresponding to desorption from a mixed ice environment. The latter is dominant for astrophysically relevant ice “thicknesses”. The ratio of the binding energies, R_BE, for pure N₂ and CO is found to be , and to be close to 1 for mixed ice fractions. The model is applied to astrophysically relevant conditions for cold pre-stellar cores and for protostars which start to heat their surroundings. The importance of treating CO desorption with zeroth rather than first order kinetics is shown. The experiments also provide lower limits of for the sticking probabilities of CO-CO, N₂-CO and N₂-N₂ ices at 14 K. The combined results from the desorption experiments, the kinetic model, and the sticking probability data lead to the conclusion that these solid-state processes of CO and N₂ are very similar under astrophysically relevant conditions. This conclusion affects the explanations for the observed anti-correlations of gaseous CO and N₂H⁺ in pre-stellar and protostellar cores.