[P-CG20] 宇宙における物質の形成と進化

[PCG20-04] Connecting chemistry in the atmosphere and the midplane of protoplanetary disks

*古家 健次¹、野村 英子¹、Lee Seokho² (1.国立天文台、2.韓国天文研究院)

キーワード：原始惑星系円盤、アストロケミストリー

Planets form and acquire their initial compositions in protoplanetary disks. According to recent observations of protoplanetary disks, gas-phase elemental abundances of carbon and oxygen in the atmosphere of some disks are lower by more than one order of magnitude than those in the interstellar medium. The rationale for the low carbon and oxygen abundances in the disk gas is suggested to be the formation of not highly volatile C- and O-bearing molecules (e.g., CO2 and H2O) and their transport to the disk midplane via turbulence followed by the freeze-out onto dust grains. Understanding the chemical connection between the gas in the disk atmosphere and solids in the midplane is crucially important to connect disk observations and planet formation.

In this talk, we will present our disk model which considers gas-ice chemistry and the vertical transport of gas and icy species. We first show that the gas in the disk atmosphere becomes N-rich with respect to carbon and oxygen with time, making the vertical gradient of N/O and N/C elemental ratios in the disk. This is because carbon and oxygen are locked up in ices in the midplane via vertical transport and ice chemistry, while nitrogen is present in the very volatile form (N2) and tends to remain in the gas phase. Secondly, we discuss how the vertical transport affects the disk midplane ice composition, in particular the D/H ratio and the 16O/18O ratio of water ice, and discuss the origin of cometary water.