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Y. Aikawa (Department of Physics, The Ohio State University), T. Umebayashi (Computing Service Center, Yamagata University), T. Nakano (Nobeyama Radio Observatory, National Astronomical Observatory of Japan), S. M. Miyama (National Astronomical Observatory of Japan)
We investigate the evolution of molecular abundances in a protoplanetary disk with viscous accretion flow by solving the reaction equations as an initial-value problem. We obtain the molecular abundances, both in the gas phase and in ice mantles of grains, as functions of time and position in the disk.
In the region of surface density \lesssim 102 g cm-2 (distance from the star \gtrsim 10 AU), cosmic rays are barely attenuated even on the midplane of the disk, and produce chemically active ions. We find that through reactions with these ions considerable amounts of CO and N2, which are initially the dominant species, are transformed into CO2, CH4 and NH3. In the regions of low temperatures, these products freeze out and accumulate in ice mantles. As the matter migrates toward inner warmer regions of the disk, some of the molecules in ice mantles evaporate. Most of these evaporated molecules are transformed into less volatile molecules by the gas-phase reactions, which then freeze out. Molecular abundances crucially depend on the temperature and thus vary with the distance from the central star.
We find that the time scale of molecular evolution depends on the ionization rate and the grain size in the disk. If these values are the same as in molecular clouds, the time scale of molecular evolution is about 106yr. The time scale is larger in the case of lower ionization rate or larger grain size.
Our results show a good qualitative agreement with molecular composition in comets. Our results also suggests that comets formed in different regions of the disk have different molecular compositions, which is consistent with the observational results (A'Hearn et al. 1995).
Finally we discuss the possibility of estimating gaseous mass, temperature distribution, and accretion velocity in the disk in future observations with radio interferometers.
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The author(s) of this abstract have provided an email address for comments about the abstract: aikawa@pacific.mps.ohio-state.edu