X-ray and UV photochemical rates of CO ices

Compared to UV radiation, X-rays contribute only minimally to the interstellar radiation field, but galactic and stellar objects can produce significant local X-ray fluxes that can impact the chemical composition of nearby gases and icy dust. The aim of this study is to provide astrochemists with laboratory data on the X-ray and UV photochemical rates of CO ice, one of the most abundant ices in the universe. We used two laboratory sources emitting X-rays (Al Kα, 1486.6 eV) and UV (Lyman-α, 10.2 eV) to measure and compare X-ray and UV photochemical rates of CO ices. We used infrared spectroscopy to probe photodesorption and photodissociation, the two processes being differentiated by covering the CO ice with an argon layer to block photodesorption. For CO ice at 10 K, we find UV photodesorption rates in line with the existing literature. At 1486.6 eV, X-ray photodesorption rates are an order of magnitude higher than with UV. We show that the X-ray absorption cross section of CO allows astrochemists to scale these X-ray photodesorption rates to the X-ray spectrum of the region of interest. Regarding the CO photodissociation, our measured X-ray rates are two orders of magnitude higher than with UV, and are also proportional to the X-ray absorption cross section. The reformation reaction balances the X-ray destruction of CO, leading to a stable state where only 25% of the initial amount of CO is destroyed, strongly limiting the destruction of CO by X-rays. When this steady state is reached, only X-ray and UV photodesorption need be considered. This study provides the data needed to model X-ray and UV photochemical processes in CO ices.