2021ApJ...922..213B

Pure acetonitrile (CH₃CN) and mixed CO:CH₃CN and H₂O:CH₃CN ices have been irradiated at 15 K with vacuum ultraviolet (VUV) photons in the 7-13.6 eV range using synchrotron radiation. VUV photodesorption yields of CH₃CN and of photoproducts have been derived as a function of the incident photon energy. The coadsorption of CH₃CN with CO and H₂O molecules, which are expected to be among the main constituents of interstellar ices, is found to have no significant influence on the VUV photodesorption spectra of CH₃CN, CHCN*, HCN, CN*, and CH₃*. Contrary to what has generally been evidenced for most of the condensed molecules, these findings point toward a desorption process for which the CH₃CN molecule that absorbs the VUV photon is the one desorbing. It can be ejected in the gas phase as intact CH₃CN or in the form of its photodissociation fragments. Astrophysical VUV photodesorption yields, applicable to different locations, are derived and can be incorporated into astrochemical modeling. They vary from 0.67(± 0.33) x 10^–5 to 2.0(± 1.0) x 10^–5 molecule photon^–1 for CH₃CN depending on the region considered, which is high compared to other organic molecules such as methanol. These results could explain the multiple detections of gas-phase CH₃CN in different regions of the interstellar medium and are well correlated to astrophysical observations of the Horsehead nebula and of protoplanetary disks (such as TW Hya and HD 163296).