Laboratory simulation of heavy-ion cosmic-ray interaction with condensed CO

Abstract

<i>Context. <i/>Within dense interstellar clouds, from their periphery to regions deep inside, ice mantles on dust grains are exposed to cosmic-ray irradiation. Various swift ions contribute from protons to iron in the keV to TeV energy range. Observations show that in some lines of sight condensed CO molecules are an important component of the ice.<i>Aims. <i/>We irradiate CO ices with Ni ions of relatively high energy (50 and 537 MeV) to simulate the effects produced by fast heavy cosmic-ray ions in interstellar grain mantles.<i>Methods. <i/>CO gas is condensed on a CsI substrate at 13 K and irradiated by 50 MeV <sup>58<sup/>Ni<sup>13+<sup/> and 537 MeV <sup>64<sup/>Ni<sup>24+<sup/> ions up to a final fluence of <i>≈<i/>1 × 10<sup>13<sup/> cm<sup>-2<sup/>, at a flux of 1 × 10<sup>9<sup/> cm<sup>-2<sup/> s<sup>-1<sup/>. The sputtering yields, the destruction rate of CO, and the rate of formation of new molecular species are measured in situ by Fourier transform infrared spectroscopy (FTIR).<i>Results. <i/>The measured CO destruction cross-sections and sputtering yields induced by Ni ions are, respectively, (i) for 50 MeV, <i>σ<i/><sub>d<sub/> = 1.0 × 10<sup>-13<sup/> cm<sup>2<sup/> and <i>Y<i/> = 7 × 10<sup>4<sup/> molecules/impact; (ii) for 537 MeV, <i>σ<i/><sub>d<sub/> = 3.0 × 10<sup>-14<sup/> cm<sup>2<sup/> and <i>Y<i/> = 5.85 × 10<sup>4<sup/> molecules/impact. Based on the present and previous results, the desorption rates induced by H, Ni, and Fe ions are estimated for a wide range of energies. The contribution of the heavy ions is found to dominate over that of protons in the interstellar medium.