2018ApJ...855...62N

In this paper, we extend the results presented in our former papers on using ortho-H₂¹⁶O line profiles to constrain the location of the H₂O snowline in T Tauri and Herbig Ae disks, to include submillimeter para-H₂¹⁶O and ortho- and para-H₂¹⁸O lines. Since the number densities of the ortho- and para-H₂¹⁸O molecules are about 560 times smaller than their ¹⁶O analogs, they trace deeper into the disk than the ortho-H₂¹⁶O lines (down to z = 0, i.e., the midplane). Thus these H₂¹⁸O 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 H₂¹⁸O and para-H₂¹⁶O 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.