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Astrophysical observations show complex organic molecules (COMs) in the gasphase of protoplanetary disks. X-rays emitted from the central young stellarobject (YSO) that irradiate interstellar ices in the disk, followed by theejection of molecules in the gas phase, are a possible route to explain theabundances observed in the cold regions. This process, known as X-rayphotodesorption, needs to be quantified for methanol-containing ices. We aim atexperimentally measuring X-ray photodesorption yields of methanol and itsphoto-products from binary mixed ices: $^{13}$CO:CH$_3$OH ice andH$_2$O:CH$_3$OH ice. We irradiated these ices at 15 K with X-rays in the 525 -570 eV range. The release of species in the gas phase was monitored byquadrupole mass spectrometry, and photodesorption yields were derived. For$^{13}$CO:CH$_3$OH ice, CH$_3$OH X-ray photodesorption yield is estimated to be10$^{-2}$ molecule/photon at 564 eV. X-ray photodesorption of larger COMs,which can be attributed to either ethanol, dimethyl ether, and/or formic acid,is detected with a yield of 10$^{-3}$ molecule/photon. When methanol is mixedwith water, X-ray photodesorption of methanol and of the previous COMs is notdetected. X-ray induced chemistry, dominated by low-energy secondary electrons,is found to be the main mechanism that explains these results. We also providedesorption yields that are applicable to protoplanetary disk environments forastrochemical models. The X-ray emission from YSOs should participate in theenrichment of the protoplanetary disk gas phase with COMs such as methanol inthe cold and X-ray dominated regions because of X-ray photodesorption frommethanol-containing ices.

astronomy and astrophysics

Complex organic molecules in protoplanetary disks: X-ray photodesorption  from methanol-containing ices. Part II -- Mixed methanol-CO and methanol-H2O  ices