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

TL;DR

This study investigates how soft X-ray irradiation causes photo-desorption in H2O:CO:NH3 ices, leading to gas-phase enrichment with complex molecules, and applies findings to protoplanetary disk models.

Contribution

It provides new measurements of photo-desorption yields for mixed ices under X-ray irradiation and explores the mechanisms involved, including substrate-mediated desorption and porosity effects.

Findings

Detection of complex molecules desorbing from ices.

Photo-desorption yield varies with photon absorption and porosity.

Application to protoplanetary disk models shows gas enrichment potential.

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.