X-ray processing of the layered ices: photodesorption is not a surface phenomenon only

Yu-Jung Chen^1*, Chao-Hui Huang¹, Li-Chieh Hsiao¹, Angela Ciaravella², Guillermo M. MuñozCaro³, Antonio Jimeńez-Escobar², Cesare Cecchi-Pestellini²

¹Physics, National Central University, Taoyuan, Taiwan
²Osservatorio Astronomico di Palermo, Istituto Nazionale di Astrofisica, Palermo, Italy
³Centro de Astrobiología, CSIC-INTA, Madrid, Spain

* Presenter:Yu-Jung Chen, email:asperchen@phy.ncu.edu.tw

The Atacama Large Millimeter Array has allowed a detailed observation of molecules in protoplanetary disks, which can evolve toward solar systems like our own. While CO, CO₂, HCO, and H₂CO are often abundant species in the cold zones of the disk, CH₃OH or CH₃CN are only found in a few regions, and more-complex organic molecules are not observed. Here we report an experimental simulation of a more astrophysically realistic ice mantle morphology, organized in a bilayered structure of a H₂O:CH₄ :NH₃ mixture, covered by a layer made of CH₃OH and CO, and exposed to soft X-rays. Young solar-type stars are powerful sources of X-rays, thousands of times brighter than our middle-aged sun, with chemical implications on the surrounding disks so far relatively unexplored. The photoproducts found desorbing from both ice layers to the gas phase during the irradiation converge with those detected in higher abundances in the gas phase of protoplanetary disks, providing important insights on the nonthermal processes that drive the chemistry in these objects. Additional similar experiments using the isotopologues ¹³CH₄ and ¹⁵NH₃ demonstrated that photodesorption is not a surface phenomenon only.

Keywords: X-ray processing, nonthermal desorption, complex organic molecules