Photodesorption of acetonitrile CH3CN in UV-irradiated regions of the Interstellar Medium: an experimental evidence

Pure acetonitrile (CH3CN) and mixed CO:CH3CN and H2O:CH3CN ices have been irradiated at 15K with Vacuum UltraViolet (VUV) photons in the 7-13.6 eV range using synchrotron radiation. VUV photodesorption yields of CH3CN and of photo-products 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 photo-dissociation 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).10^{-5} to 2.0(+/-1.0).10^{-5} molecule/photon 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).