CANDIDATE WATER VAPOR LINES TO LOCATE THE H2O SNOWLINE THROUGH HIGH-DISPERSIONSPECTROSCOPIC 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).