Photo-desorption of H2O:CO:NH3 circumstellar ice analogs: Gas-phase enrichment

Abstract

We study the photo-desorption occurring in H${}_2$O:CO:NH${}_3$ ice mixtures irradiated with monochromatic (550 and 900 eV) and broad band (250--1250 eV) soft X-rays generated at the National Synchrotron Radiation Research Center (Hsinchu, Taiwan). We detect many masses photo-desorbing, from atomic hydrogen (m/z = 1) to complex species with m/z = 69 (e.g., C${}_3$H${}_3$NO, C${}_4$H${}_5$O, C${}_4$H${}_7$N), supporting the enrichment of the gas phase. At low number of absorbed photons, substrate-mediated exciton-promoted desorption dominates the photo-desorption yield inducing the release of weakly bound (to the surface of the ice) species; as the number of weakly bound species declines, the photo-desorption yield decrease about one order of magnitude, until porosity effects, reducing the surface/volume ratio, produce a further drop of the yield. We derive an upper limit to the CO photo-desorption yield, that in our experiments varies from 1.4 to 0.007 molecule photon${}^{-1}$ in the range $\sim 10^{15}-10^{20}$~absorbed photons cm${}^{-2}$. We apply these findings to a protoplanetary disk model irradiated by a central T~Tauri star.

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• Impact of an Active Sgr A* on the Synthesis of Water and Organic Molecules Throughout the Milky Way
• Interstellar carbonaceous dust erosion induced by X-ray irradiation of water ice in star-forming regions
• Indirect X-ray photodesorption of 15N2 and 13CO from mixed and layered ices