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Abstract:
Carbon abundances in rocky planets play an important role in shaping their surface environments. Additionally, the C/O ratio in the atmospheres of gas giant exoplanets may reveal their birthplace. Therefore, it is crucial to investigate how carbon is distributed in both the gas and solid phases of protoplanetary disks. However, despite active discussion in recent years, this remains unclear due to uncertainties about the forms of carbon-bearing species in the solid phase and the motion of solids in protoplanetary disks. In this talk, I will present the current understanding of carbon abundance and its time evolution in protoplanetary disks for both the gas and solid phases. In the first half, I will address the unresolved problem of the solid phase: the Earth’s carbon deficit. The Earth is significantly depleted in carbon compared to comets and interstellar dust, which contain abundant refractory carbonaceous solids, such as organics and amorphous hydrocarbons. Since the disk temperature around Earth’s orbit is not high enough to vaporize these carbonaceous solids, this significant depletion remains an unsolved problem. I will discuss several models proposed to explain this deficit, focusing on thermal decomposition of organics and gasification by far-ultraviolet radiation (photolysis), as well as the limitations of these models. In the second half, I will examine the carbon abundance in the gas phase of disks. While classical disk models only considered the sublimation of volatile carbon species such as CO and CO₂, recent studies have shown that degassing from refractory carbonaceous solids significantly contributes to the gas-phase carbon abundance. I will focus on the effects of degassing caused by photolysis and thermal decomposition of these solids.