X-ray photodesorption of complex organic molecules in protoplanetary disks

I. Acetonitrile CH₃CN

¹ Sorbonne Université, Observatoire de Paris, Université PSL, CNRS, LERMA, 75005 Paris, France
e-mail: romain.basalgete@sorbonne-universite.fr
² Université Paris-Saclay, CNRS, ISMO, 91405 Orsay, France

Received: 21 February 2023
Accepted: 8 June 2023

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.