Observationally locating the position of the H2O snowline in protoplanetary disks is crucial for understanding the planetesimal and planet formation processes, and the origin of water on the earth. The velocity profiles of emission lines from disks are usually affected by Doppler shift due to Keplerian rotation. Therefore, the line profiles are sensitive to the radial distribution of the line-emitting regions.  In our previous studies (Notsu et al. 2016, 2017, 2018), on the basis of our disk chemical modeling and water line calculations, we found that the water lines with small Einstein A coefficients and relatively high upper state energies are dominated by emission from the hot midplane region inside the H2O snowline, and therefore through analyzing their line profiles the position of the H2O snowline can be located. There are several best candidate water lines that trace the position of the H2O snowline within the wavelength coverage of ALMA and GREX-PLUS.

Recently, we got the upper limit fluxes of several water lines from the disk around the Herbig Ae star HD163296, using ALMA (Notsu et al. 2019). We compared the upper limit fluxes with the values calculated by our disk model calculations, and we constrained the line emitting region and the dust opacity from the observations. Future observations of the submillimeter water lines with longer observation time with ALMA expect to clarify the position of the H2O snowline in several Herbig disks and younger disks.

According to our modeling results, the values of the total line fluxes tend to be larger as the wavelengths of the water lines become shorter. Since GREX-PLUS will have a high sensitivity in the Q band (mid-infrared wavelengths), we predict not only successful detections of H2(16)O 17.75um line (= the water snowline tracer line) for several Herbig Ae and T Tauri disks, but also suggest the possibility of a survey of H2O snowline locations in many Herbig Ae disks and T Tauri disks in nearby star-forming regions for the first time (See also Kamp et al. 2021).

In this talk, first we will briefly review recent topics about molecular line observations in protoplanetary disks, and then present the results of our studies about water snowline. In the last, we will also introduce an overview of the protoplanetary disk science which are expected to be realized with future infrared space telescope (GREX-PLUS).