title: Chemical Reactions in Protoplanetary Disks and Possibility of Detecting H2O Snowline Using Spectroscopic Observations

authors: Shota Notsu, Hideko Nomura, Daiki Ishimoto

abstract:  Inside H2O snowline in protoplanetary disks, H2O evaporates from the grain surface into gas. On the other hand, it is frozen out on the grain surface in the cold region beyond H2O snowline. In the disks around solar-mass T-tauri stars. H2O snowline is thought to exist at a few AU from the central star. Therefore, it is difficult to detect H2O snowline of exoplanetary systems by imaging observations, since their spatial resolution is insufficient.

 We have proposed the method of detecting H2O snowline by analyzing the velocity profiles of H2O line spectra that will be obtained by high dispersion spectroscopic observations.
First, we calculate chemical reactions and investigate abundance distribution of H2O gas. We confirmed that the abundance of H2O is high not only in the inner region of H2O snowline near the equatorial plane but also in the hot surface layer of outer disk.
Second, we calculate the velocity profiles of H2O emission lines, and found that we can obtain the information of H2O snowline through investigating the profiles of some emissions that have small Einstein A coefficient and large excitation energy. The wavelengths of the useful H2O emission lines range from mid-infrared to sub-millimeter. 

 In addition, we investigate the effect of grain surface reactions. The abundance of water vapor increases inside H2O snowline, while it decreases in the hot surface layer of outer disk. Hence, the line fluxes of H2O transitions with small Einstein A coefficient and high excitation energy become higher. It appears more significantly in the lines at the wavelengths of sub-millimeter than those at infra-red.