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

Abstract

Pure acetonitrile (CH 3 CN) and mixed CO:CH 3 CN and H 2 O:CH 3 CN 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 CH 3 CN and of photo-products have been derived as a function of the incident photon energy. The coadsorption of CH 3 CN with CO and H 2 O 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 CH 3 CN, CHCN•, HCN, CN• and CH 3 •. Contrary to what has generally been evidenced for most of the condensed molecules, these findings point toward a desorption process for which the CH 3 CN molecule that absorbs the VUV photon is the one desorbing. It can be ejected in the gas phase as intact CH 3 CN 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 CH 3 CN 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 CH 3 CN 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).