[P-PS24_1PM2] Origin and evolution of materials in space

[PPS24-10] Hydrogen addition reactions of C₂H₂ on cold grains; clue to the formation mechanism of cometary C₂H₆

*Hitomi KOBAYASHI¹, Naoki WATANABE², Hiroshi HIDAKA², Tetsuya HAMA², Yoji WATANABE¹, Hideyo KAWAKITA¹ (1.Koyama Astronomical Observatory, Kyoto Sangyo Univeristy, 2.Institute of Low temperature Science, Hokkaido University)

Keywords:molecular formation, grain surface chemistry, Inter Stellar Medium

Volatiles incorporated into comets were formed in the pre-solar molecular cloud and probably chemically altered in the proto-planetary disk of the Sun. Although physico-chemical evolution from a molecular cloud to the disk is basically understood, detailed evolutional processes are still in debate; e.g., the fraction of the materials originated in the molecular cloud incorporated into the disk without physico-chemical alterations (some fraction of materials might sublimate via accretion shock) and physical conditions (temperature, densities of materials, etc.). To reveal those links, we focused on the molecules formed through grain surface reactions, which occurred under quite low temperature conditions like 10K. We discuss the origin of such molecules in comets (icy small body of the Solar system), which might preserve the information about chemical and physical conditions of proto-planetary disk. Cometary ethane (C₂H₆) and acetylene (C₂H₂) have been observed in multiple comets since 1996 and their abundances relative to H2O (the major component of cometary ices) is ~10^-3 but with variations. This variation might be caused by the difference in the mixing ratios between the materials originated in the molecular cloud and the disk-processed materials. C₂H₆ has never been detected in the molecular cloud and the formation mechanism of C₂H₆ detected in comets is still in debate. One of the candidates of formation reactions of C₂H₆ is the hydrogen addition reaction of C₂H₂ on the cold grain surface (C₂H₂ -> C₂H₃ -> C₂H₄ -> C₂H₅ -> C₂H₆). In the previous experimental studies, those reactions were evaluated qualitatively and it was concluded that the reaction from C₂H₄ to C₂H₆ occurred more rapidly than the reactions from C₂H₂ to C₂H₄ and it would be a reason for the nondetection of C₂H₄. To investigate these reactions more quantitatively in realistic conditions for molecular clouds, we performed the laboratory measurements of hydrogen addition reactions of C₂H₂ and C₂H₄ on the amorphous solid water (ASW), respectively. The experiments were conducted by using laboratory setup for surface reaction in interstellar environment (LASSIE) at the institute of low temperature science, Hokkaido University3. A cryogenic aluminum substrate is located in the center of the main chamber and surrounded by a large copper shroud connected to a liquid-nitrogen reservoir. Atomic hydrogen used for the reactions were produced by the dissociation of H2 molecules in microwave-induced plasma. The kinetic temperature of hydrogen atoms were ~120 K and the H-atom flux was ~10¹³ cm^-2 s^-1. The samples of pure solid C₂H₂, C₂H₄, and those on ASW were produced on the substrate at 10, 15 and 20K. Infrared absorption spectra of the ices were measured by FTIR before and during the exposure of H-atom. Our measurements show basically the same trend as shown in the previous studies. We will discuss the temperature and thickness dependence of the time constant for the sample ices in the poster.