Candidate Water Vapor Lines to Locate the H2O Snowline through High-dispersion Spectroscopic Observations. III. Submillimeter H2 16O and H2 18O Lines

In this paper, we extend the results presented in our former papers on using ortho- line profiles to constrain the location of the H2O snowline in T Tauri and Herbig Ae disks, to include submillimeter para- and ortho- and para- lines. Since the number densities of the ortho- and para- molecules are about 560 times smaller than their 16O analogs, they trace deeper into the disk than the ortho- lines (down to z = 0, i.e., the midplane). Thus these lines are potentially better probes of the position of the H2O snowline at the disk midplane, depending on the dust optical depth. The values of the Einstein A coefficients of submillimeter candidate water lines tend to be lower (typically <10−4 s−1) than infrared candidate water lines. Thus in the submillimeter candidate water line cases, the local intensity from the outer optically thin region in the disk is around 104 times smaller than that in the infrared candidate water line cases. Therefore, in the submillimeter lines, especially and para- lines with relatively lower upper state energies (~a few 100 K) can also locate the position of the H2O snowline. We also investigate the possibility of future observations with ALMA to identify the position of the water snowline. There are several candidate water lines that trace the hot water gas inside the H2O snowline in ALMA Bands 5–10.