Theoretical Water Binding Energy Distribution and Snowline in Protoplanetary Disks

Water is one of the most important and abundant molecules in star-forming regions. In protoplanetary disks, where planets and comets form, H2O is in a gas or solid form, depending on the dust temperature, i.e., the distance from the center and its binding energy (BE). Not surprisingly, several experimental and theoretical studies of the H2O BE have been published. We report new ab initio calculations carried out on a large model of interstellar ice, where we identified 144 different adsorption sites. The BE associated with those sites ranges between 14.2 kJ mol(-1) (1705 K) and 61.6 kJ mol(-1) (7390 K). The distribution of the computed BEs as a function of BE follows a Gaussian peaked at 35.4 kJ mol(-1) (4230 K) with a standard deviation of 9.7 kJ mol(-1) (1160 K). The computed pre-exponential factor (& nu;) ranges between 9 x 10(12) and 6 x 10(14) s(-1). We evaluated the impact of the newly calculated BE and & nu; distributions on the snowline of a generic protoplanetary disk. We found that the region where water is frozen onto the ice is much smaller (a factor of 10 smaller radius) than that computed with the single BE (5600 K) and & nu; (2 x 10(12) s(-1)) values commonly adopted by astrochemical models. Besides, & SIM;10% of water remains frozen in relatively warm (& SIM;150 K) regions, where the single BE and & nu; model would predict a full release of the ice in the gas phase. This last aspect may have an impact on the quantity trapped in the planetesimals eventually forming rocky planets.