Cosmic ray impact on astrophysical ices: laboratory studies on heavy ion irradiation of methane

¹ Departamento de Disciplinas Básicas e Gerais, CEFET-RJ, Av. Maracanã 229, 20271-110, Rio de Janeiro, RJ, Brazil
e-mail: abarros@if.ufrj.br
² Departamento de Física, Pontifícia Universidade Católica do Rio de Janeiro, Rua Marquês de São Vicente 225, 22453-900 Rio de Janeiro, RJ, Brazil
³ Grupo de Física e Astronomia, Instituto Federal do Rio de Janeiro (IFRJ), Rua Lucio Tavares 1045, 26530-060 Nilópolis, RJ, Brazil
⁴ Centre de Recherche sur les Ions, les Matériaux et la Photonique CIMAP-GANIL (CEA-CNRS-ENSICAEN-UCBN), BP 5133, Boulevard Henri Becquerel, 14070 Caen Cedex 05, France

Received: 28 October 2010
Accepted: 18 April 2011

Abstract

Laboratory data of CH₄ ice radiolysis promoted by fast heavy ions are obtained by infrared spectroscopy (FTIR). CH₄ molecules are condensed on a CsI substrate at 15 K, and the ice layer is bombarded by 220 MeV ¹⁶O⁷⁺ ion beam. The ice thickness is thin enough to be traversed by projectiles at constant velocity close to the equilibrium charge state. The induced CH₄ dissociation gives rise to the formation of molecular species CH₃, C₂H₂, C₂H₄, C₂H₆, and C₃H₈. Their formation and dissociation cross sections are determined. C₂H₆ represent the most abundant daughter molecules. The carbon budget analysis of CH₄ and its radiolysis products shows that the column density of carbon atoms contained in the methane destroyed during ion irradiation is 30 − 50% greater than the sum for the column densities of the newly formed species. As an astrophysical implication, the current results allow estimation of chemical reaction rates in ices covering interstellar grains.