Molecular evolution in planet-forming circumstellar disks

Abstract

We have investigated the evolution of molecular abundance in circumstellar disks around young low-mass stars, which are considered to be the formation sites of planetary systems. Adopting the standard accretion disk model, we investigated molecular evolution mainly in the accretion phase. In the region of surface density less than 10² g cm^-2 (distance from the star 10 AU), cosmic rays are barely attenuated, even in the midplane of the disk, and produce chemically active ions such as He⁺ and H₃⁺. We found that a considerable amount of CO and N₂, the initial dominant components of the disk, is transformed into CO₂, CH₄, NH₃ and HCN through reactions with these ions. Where the temperature is low enough for these products to freeze onto grains, they are selectively ‘locked up’ and accumulate in the ice mantle. As the matter accretes towards inner warmer regions, the ice mantle evaporates. The desorbed molecules, such as CH₄, are transformed into larger and less volatile molecules by reactions in the gas phase. The molecular abundance, both in the gas phase and in the ice mantle, depends crucially on the temperature and thus varies significantly with the distance from the central star. If the ionization rate and the grain size in the disk are the same as those in molecular clouds, the timescale of the molecular evolution, in which CO and N₂ are transformed into other molecules is, ca. 10⁶ years, slightly less than the life time of the disk. The timescale of molecular evolution is less for higher ionization rates and greater for lower ionization rates or larger grain size. We have compared our results with the molecular composition in comets, the most primitive bodies in our solar system. The molecular abundance derived from our model reproduces the coexistence of oxidized ice and reduced ice, as observed in comets. Our model also suggests that comets formed in different regions of the disk will have different molecular compositions.

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