Water is the main constituent of interstellar ices, and it plays a key role in the evolution of many regions of the interstellar medium, from molecular clouds to planet-forming disks1. In cold regions of the interstellar medium, water is expected to be completely frozen out onto the dust grains. Nonetheless, observations indicate the presence of cold water vapour, implying that non-thermal desorption mechanisms are at play. Photodesorption by ultraviolet photons has been proposed to explain these observations2,3, with the support of extensive experimental and theoretical work on ice analogues4,5,6. In contrast, photodesorption by X-rays, another viable mechanism, has been little studied. The potential of this process to desorb key molecules such as water, intact rather than fragmented or ionized, remains unexplored. We experimentally investigated X-ray photodesorption from water ice, monitoring all desorbing species. We found that desorption of neutral water is efficient, while ion desorption is minor. We derived yields that can be implemented in astrochemical models. These results open up the possibility of taking into account the X-ray photodesorption process in the modelling of protoplanetary disks or X-ray-dominated regions.
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We thank C. Walsh for insights on X-rays in protoplanetary disks, D. Lis for comments on the paper and P. Marie-Jeanne for technical support. We acknowledge SOLEIL for the provision of synchrotron radiation facilities under project 20161406, and we thank N. Jaouen and the SEXTANTS team for their help on the beamline. This work was supported by the Programme National ‘Physique et Chimie du Milieu Interstellaire’ (PCMI) of CNRS/INSU with INC/INP co-funded by CEA and CNES. Financial support from LabEx MiChem, part of the French state funds managed by the ANR within the investissements d’avenir programme under reference ANR-11-10EX-0004-02, and by the Ile-de-France region DIM ACAV programme, is gratefully acknowledged. This work was done in collaboration with and through financial support from the European Organization for Nuclear Research (CERN) under collaboration agreement KE3324/TE.
The authors declare no competing interests.
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Dupuy, R., Bertin, M., Féraud, G. et al. X-ray photodesorption from water ice in protoplanetary disks and X-ray-dominated regions. Nat Astron 2, 796–801 (2018). https://doi.org/10.1038/s41550-018-0532-y