X-Ray Photo-desorption of H2O:CO:NH3 Circumstellar Ice Analogs: Gas-phase Enrichment

A. Jimenez-Escobar, A. Ciaravella, C. Cecchi-Pestellini, C.-H. Huang, N.-E. Sie, Y.-J. Chen, G.M. Munoz Caro. 2018. X-Ray Photo-desorption of H2O:CO:NH3 Circumstellar Ice Analogs: Gas-phase Enrichment. Astrophysical Journal 868, 1, DOI: 10.3847/1538-4357/aae711

We study the photo-desorption occurring in H2O:CO:NH3 ice mixtures irradiated with monochromatic (550 and 900 eV) and broadband (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., C3H3NO, C4H5O, C4H7N), supporting the enrichment of the gas phase. At low numbers 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 decreases 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, which in our experiments varies from 1.4 to 0.007 molecules photon-1 in the range similar to 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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