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Evolution of a snow line in an optically-thick protoplanetary disk isinvestigated with numerical simulations. The ice-condensing region in the diskis obtained by calculating the temperature and the density with the 1+1Dapproach. The snow line migrates as the mass accretion rate (\dot{M}) in thedisk decreases with time. Calculations are carried out from an early phase withhigh disk accretion rates (\dot{M} \sim 10^{-7} M_sun/yr) to a later phase withlow disk accretion rates (\dot{M} \sim 10^{-12} M_sun/yr) using the samenumerical method. It is found that the snow line moves inward for \dot{M} >10^{-10} M_sun/yr, while it gradually moves outward in the later evolutionphase with \dot{M} < 10^{-10} M_sun/yr. In addition to the silicate opacity,the ice opacity is taken into consideration. In the inward migration phase, theadditional ice opacity increases the distance of the snow line from the centralstar by a factor of 1.3 for dust grains < 10 micro-meter in size and 1.6 for >100 micro-meter. It is inevitable that the snow line comes inside the Earth'sorbit in the course of the disk evolution, if the alpha viscosity parameter isin a range 0.001-0.1, the dust-to-gas mass ratio is higher than a tenth of thesolar abundance value, and the dust grains are smaller than 1 mm. The formationof water-devoid planetesimals in the terrestrial planet region seems to bedifficult throughout the disk evolution, which imposes a new challenge toplanet formation theory.

Evolution of Snow Line in Optically Thick Protoplanetary Disks: Effects of Water Ice Opacity and Dust Grain Size