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 H 2 O 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 16 O 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 H 2 O 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 10 4 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 H 2 O 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 H 2 O snowline in ALMA Bands 5–10.