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

TL;DR

This study experimentally measures X-ray photodesorption yields of methanol and complex organic molecules from mixed ices at 15 K, revealing the process's significance in enriching protoplanetary disk gas with COMs.

Contribution

It provides the first quantitative measurements of X-ray photodesorption yields for methanol and related COMs from mixed ices, informing astrochemical models of protoplanetary disks.

Findings

Methanol X-ray photodesorption yield is approximately 10^{-2} molecules per photon.

Detection of larger COMs with yields around 10^{-3} molecules per photon.

X-ray photodesorption is suppressed in water-rich ices.

Abstract

Astrophysical observations show complex organic molecules (COMs) in the gas phase of protoplanetary disks. X-rays emitted from the central young stellar object (YSO) 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. We aim at experimentally measuring X-ray photodesorption yields of methanol and its photo-products from binary mixed ices: $^{13}$CO:CH$_3$OH ice and H$_2$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 by quadrupole mass spectrometry, and photodesorption yields were derived. For $^{13}$CO:CH$_3$OH ice, CH$_3$OH X-ray photodesorption yield is estimated to be 10$^{-2}$ 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 mixed with water, X-ray photodesorption of methanol and of the previous COMs is not detected. X-ray induced chemistry, dominated by low-energy secondary electrons, is found to be the main mechanism that explains these results. We also provide desorption yields that are applicable to protoplanetary disk environments for astrochemical models. The X-ray emission from YSOs should participate in the enrichment of the protoplanetary disk gas phase with COMs such as methanol in the cold and X-ray dominated regions because of X-ray photodesorption from methanol-containing ices.