Complex Organic Molecules (COMs) in protoplanetary disks : X-ray photodesorption from pure and binary CH3OH:(H2O/CO) ice

Abstract:

The most recent astrophysical observations show the presence of Complex Organic Molecules (COMs), among which methanol, formaldehyde and formic acid, in the gas phase of protoplanetary disks. The very cold temperatures in the concerned regions fail to explain these puzzling detections, since the majority of these species should be accreted onto the surface of the dust grains. Among several non-thermal processes that may be at play, X-rays, emitted from the central Young Stellar Object (YSO), irradiating interstellar ices in the disk, followed by the ejection of molecules in the gas phase, is a possible route to explain the abundances observed. This process, known as X-ray photodesorption, has already proven to be efficient for "simple" species such as H₂O and is potentially competing with UV photodesorption processes. However, its role in the origin of gaseous COMs is still uncertain and needs to be quantified.

We aim at experimentally determine X-ray photodesorption yields of methanol and other species for different ices : pure CH₃OH ice, ¹³CO:CH₃OH ice and H₂O:CH₃OH ice. We irradiated the previous ices, at 15 K, by monochromatic X-rays in the 525 - 570 eV range from the SEXTANTS beamline of the SOLEIL synchrotron facility. The release of species in the gas phase is monitored by quadrupole mass spectrometry, and photodesorption yields of several molecules are computed.

We have measured methanol X-ray photodesorption yields from pure methanol ice and from ¹³CO :CH₃OH ice, as well as other various species including ethanol, dimethyl ether and/or formic acid. When methanol is mixed in water mantles, X-ray photodesorption of methanol and previous COMs is not detected. X-ray induced electron-stimulated desorption (XESD), including X-ray induced-chemistry, dominated by low energy secondary electrons, is found to be the main mechanism explaining the measured photodesorption yields. X-ray photodesorption is found to be as as efficient as UV photons when taking into account the differences in the local X-ray and UV field in protoplanetary disks.

These results highlight the fact that X-ray emission from YSOs should participate to the enrichment of the protoplanetary disk gas phase with molecules of astrochemical interest in the cold and X-ray dominated regions, due to X-ray photodesorption from methanol containing ices.