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

Abstract

Context. 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. Aims. 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. Methods. CO gas is condensed on a CsI substrate at 13 K and irradiated by 50 MeV 58 Ni 13+ and 537 MeV 64 Ni 24+ ions up to a final fluence of ≈1 × 10 13 cm −2 , at a flux of 1 × 10 9 cm −2 s −1 . 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). Results. The measured CO destruction cross-sections and sputtering yields induced by Ni ions are, respectively, (i) for 50 MeV, σ d = 1.0 × 10 −13 cm 2 and Y = 7 × 10 4 molecules/impact; (ii) for 537 MeV, σ d = 3.0 × 10 −14 cm 2 and Y = 5.85 × 10 4 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.

New experimental sublimation energy measurements for some relevant astrophysical ices

“…Seperuelo perform a series of experiments where the desorption rate induced by H, Ni and Fe ions are estimated, concluding that the effect of heavy ions is even more important than the effect of protons in the ISM. The works performed by Muñoz-Caro et al (2010) andSeperuelo Duarte et al (2010), are relevant for the ISM because, even though CO can be mixed with water ice, from the absorption profile studied by Ehrenfreund et al (1996), it is possible to deduce that CO could be embedded in a nearly separate ice phase.…”

New results on thermal and photodesorption of CO ice using the novel InterStellar Astrochemistry Chamber (ISAC)

“…Tafalla et al (2004) find 2.5 and 7.8 × 10 4 cm −3 for the minimum densities required for depletion of CO gas. In addition to these works on the study of photodesorption, others have explored the possibility of CO desorption induced by cosmic rays, which can play an important role inside dense clouds (Seperuelo Duarte et al 2010, and references therein). The CO ice absorption profile near 2140 cm −1 shows that it can be embedded in a nearly pure separate ice phase .…”

Radiolysis and sputtering of carbon dioxide ice induced by swift Ti, Ni, and Xe ions

“…Deep within dense clouds, galactic cosmic rays are the dominant source of ionizing radiation, either due to direct interaction with the icy grains, or by inducing secondary UV photons from (gas phase) hydrogen. The heavy ion fraction in cosmic rays (most have energies between 1 MeV/u and 100 MeV/u) yields non negligible contributions to sputtering and radiolysis due to their high electronic energy loss, even if protons and alpha particles are more abundant [6,7]. For recent reviews, we refer the readers to Refs.…”