Complex organic molecules in protoplanetary disks: X-ray photodesorption from methanol-containing ices

Abstract

Context. Astrophysical observations show complex organic molecules (COMs) in the gas phase of protoplanetary disks. X-rays emitted from the central young stellar object that irradiate interstellar ices in the disk, followed by the ejection of molecules in the gas phase, are a possible route to explain the abundances observed in the cold regions. This process, known as X-ray photodesorption, needs to be quantified for methanol-containing ices. This Paper I focuses on the case of X-ray photodesorption from pure methanol ices. Aims. We aim at experimentally measuring X-ray photodesorption yields (in molecule desorbed per incident photon, displayed as molecule/photon for more simplicity) of methanol and its photo-products from pure CH3OH ices, and to shed light on the mechanisms responsible for the desorption process. Methods. We irradiated methanol ices at 15 K with X-rays in the 525–570 eV range from the SEXTANTS beam line of the SOLEIL synchrotron facility. The release of species in the gas phase was monitored by quadrupole mass spectrometry, and photodesorption yields were derived. Results. Under our experimental conditions, the CH3OH X-ray photodesorption yield from pure methanol ice is ~10−2 molecule/photon at 564 eV. Photo-products such as CH4, H2CO, H2O, CO2, and CO also desorb at increasing efficiency. X-ray photodesorption of larger COMs, which can be attributed to either ethanol, dimethyl ether, and/or formic acid, is also detected. The physical mechanisms at play are discussed and must likely involve the thermalization of Auger electrons in the ice, thus indicating that its composition plays an important role. Finally, we provide desorption yields applicable to protoplanetary disk environments for astrochemical models. Conclusions. The X-rays are shown to be a potential candidate to explain gas-phase abundances of methanol in disks. However, more relevant desorption yields derived from experiments on mixed ices are mandatory to properly support the role played by X-rays in nonthermal desorption of methanol (see Paper II).

Indirect x-ray photodesorption of N215 and CO13 from mixed and layered ices

“…More recently, the desorption induced by the irradiation of X-rays, referred to as X-ray photodesorption, was also highlighted as a significant process for ices composed of simple (Dupuy et al, 2018(Dupuy et al, , 2021b and complex molecules (Jiménez-Escobar et al, 2018;Ciaravella et al, 2020;Basalgète et al, 2021a). More importantly, it was suggested that X-ray photodesorption of neutral molecules was strongly dependent on the ice composition via indirect desorption mechanisms (Basalgète et al, 2021b).…”

Cryogenic Chemistry and Quantitative Non‐Thermal Desorption from Pure Methanol Ices: High‐Energy Electron versus X‐Ray Induced Processes

“…The irradiation experiments allowed us to evaluate the contribution of Auger electrons generated in the ice upon X-Ray absorption. By comparing the yields measured under X-ray irradiation, [8] the dominant role played by XESD and the thermalization of the generated Auger electron is clearly demonstrated. Methanol ices, typically 70 to 100 monolayers (ML) thick, were grown by vapour deposition on a cold substrate (23 K) and irradiated with 505 eV electrons.…”

“…Comparison of relative desorption yields between ESD (present work) and X-Ray photodesorption. [8] Each data set is normalised to the CH3OH yield. Yields for the black and blue data set shown were calculated using the partial ionisation cross-sections for to CH4 and formic acid (FA), respectively.…”

“…These values demonstrate that complex chemistry is induced within the film at comparable global efficiency. The yields relative to methanol (Γ0 X/Γ0 CH3OH) for selected products derived from the present work are compared to the ones from X-Ray photodesorption experiments [8] in Figure 3, right panel. In general, the chemistry is replicated as exemplified by the good match of relative yields of H2CO and m/z 46 contributors (CH3OCH3, CH3CH2OH, and HCOOH).…”

X-ray induced desorption and photochemistry in CO ice

“…Cosmic-ray induced desorption from CO has also been studied from early on 17,18 and more recently with heavy ions 19 . In several recent papers, we have explored the a Sorbonne Université, Observatoire de Paris, Université PSL, CNRS, LERMA, F-75005, Paris, France; E-mail: dupuy@fhi.mpg.de b Institut de Chimie Physique, UMR 8000, CNRS, Université Paris-Saclay, 91405, Orsay, France c Laboratori Nazionali di Frascati (LNF)-INFN I-00044 Frascati d CERN, CH-1211 Geneva 23, Switzerland astrophysical implications 20 and the details 21 of the X-ray photodesorption process from water ice, as well as X-ray photodesorption from methanol in pure 22 and mixed ices 23 . Other authors also studied X-ray photodesorption from mixed ices 24,25 .…”