Candidate Water Vapor Lines to Locate the H${}_2$O Snowline through High-Dispersion Spectroscopic Observations I. The Case of a T Tauri Star

Abstract

Inside the H${}_2$O snowline of protoplanetary disks, water evaporates from the dust-grain surface into the gas phase, whereas it is frozen out on to the dust in the cold region beyond the snowline. H${}_2$O 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 H${}_2$O snowline as the surface that divides the regions between rocky and gaseous giant planet formation. Thus observationally measuring the location of the H${}_2$O 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 H${}_2$O snowline through future high-dispersion spectroscopic observations. First, we calculate the chemical composition of the disk and investigate the abundance distributions of H${}_2$O gas and ice, and the position of the H${}_2$O snowline. We confirm that the abundance of H${}_2$O gas is high not only in the hot midplane region inside the H${}_2$O snowline but also in the hot surface layer of the outer disk. Second, we calculate the H${}_2$O line profiles and identify those H${}_2$O lines which are promising for locating the H${}_2$O snowline: the identified lines are those which have small Einstein $A$ coefficients and high upper state energies. The wavelengths of the candidate H${}_2$O lines range from mid-infrared to sub-millimeter, and they overlap with the regions accessible to ALMA and future mid-infrared high dispersion spectrographs (e.g., TMT/MICHI, SPICA).

33 pages, 9 figures, and 2 tables are contained in this paper. It was received by The Astrophysical Journal (ApJ) on December 30th, 2015, and was accepted on June 16th, 2016

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