X-ray photodesorption of complex organic molecules in protoplanetary disks

Abstract

Context. X-rays emitted from pre-main-sequence stars at the center of protoplanetary disks can induce nonthermal desorption from interstellar ices populating the cold regions of the disk. This process, known as X-ray photodesorption, needs to be quantified for complex organic molecules (COMs), including acetonitrile CH3CN, which has been detected in several disks. Aims. The purpose of this work is to experimentally estimate the X-ray photodesorption yields of neutral species from pure CH3CN ices and from interstellar ice analogs for which CH3CN is mixed either in a CO-dominated ice or in a H2O-dominated ice. Methods. The ices, grown in an ultrahigh vacuum chamber, were irradiated at 15 K by soft X-rays from synchrotron light (SOLEIL synchrotron) in the N K edge region (395–420 eV) and in the O K edge region (530–555 eV). X-ray photodesorption was probed in the gas phase via quadrupole mass spectrometry by monitoring the changes in the mass signals due to the X-ray irradiation of the ices. X-ray photodesorption yields were derived from the mass signals and were extrapolated to higher X-ray energies in order to provide astrophysical yields adapted to astrochemical models. Results. X-ray photodesorption of the intact CH3CN is detected from pure CH3CN ices and from mixed 13CO:CH3CN ices, with an experimental yield of about 5 × 10−4 molecules photon−1 at 560 eV. When mixed in H2O-dominated ices, X-ray photodesorption of the intact CH3CN at 560 eV is below its detection limit, which is 10−4 molecules photon−1. Yields associated with the desorption of HCN, CH4, and CH3 are also provided. The derived astrophysical yields significantly depend on the local conditions expected in protoplanetary disks, that is, on the ice composition and on the local X-ray irradiation spectrum. They vary from ~10−4 to ~10−6 molecules photon−1 for the X-ray photodesorption of intact CH3CN from CO-dominated ices. Only upper limits varying from ~5 × 10−5 to ~5 × 10−7 molecules photon−1 could be derived for the X-ray photodesorption of intact CH3CN from H2O-dominated ices. Conclusions. X-ray photodesorption of intact CH3CN from interstellar ices might in part explain the abundances of CH3CN observed in protoplanetary disks. The desorption efficiency is expected to vary with the local physical conditions, hence with the disk region considered.