Candidate Water Vapor Lines to Locate the H₂O Snowline through High-dispersion Spectroscopic Observations. I. The Case of a T Tauri Star

Inside the H2O snowline of protoplanetary disks, water evaporates from the dust-grain surface into the gas phase, whereas it is frozen out onto the dust in the cold region beyond the snowline. H2O ice enhances the solid material in the cold outer part of a disk, which promotes the formation of gas-giant planet cores. We can regard the H2O snowline as the surface that divides the regions between rocky and gaseous giant planet formation. Thus observationally measuring the location of the H2O snowline is crucial for understanding the planetesimal and planet formation processes, and the origin of water on Earth. In this paper, we find candidate water lines to locate the H2O snowline through future high-dispersion spectroscopic observations. First, we calculate the chemical composition of the disk and investigate the abundance distributions of H2O gas and ice, and the position of the H2O snowline. We confirm that the abundance of H2O gas is high not only in the hot midplane region inside the H2O snowline but also in the hot surface layer of the outer disk. Second, we calculate the H2O line profiles and identify those H2O lines that are promising for locating the H2O snowline: the identified lines are those that have small Einstein A coefficients and high upper state energies. The wavelengths of the candidate H2O lines range from mid-infrared to sub-millimeter, and they overlap with the regions accessible to the Atacama Large Millimeter/sub-millimeter Array and future mid-infrared high-dispersion spectrographs (e.g., TMT/MICHI, SPICA).