Observationally measuring the location of the H2O snowline in protoplanetary disks is crucial for understanding the planetesimal and planet formation processes, and the origin of water on rocky planets including the Earth. However, it is challenging to locate the position of the H2O snowline by using current imaging facilities, since the spatial resolution of existing telescopes is insufficient. 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 work (e.g., Notsu et al. 2016, ApJ, 827, 113; 2017, ApJ, 836, 118), we found candidate water lines to locate the position of the H2O snowline through high-dispersion spectroscopic observations. First, we calculated the chemical composition of the disks around a T Tauri star and a Herbig Ae star using chemical kinetics. We confirmed that the abundance of H2O gas is high not only in the hot midplane region inside the H2O snowline but also in the hot surface layer and the photodesorbed region of the outer disk. The position of the H2O snowline in the Herbig Ae disk is further from the central star compared to that in the T Tauri disk. Second, we calculated the H2O line profiles and identified that H2O 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 profiles the position of the H2O snowline can be located. The wavelengths of the the candidate H2O lines to locate the position of the H2O snowline range from mid-infrared to sub-millimeter, and the total line fluxes tend to increase with decreasing wavelengths, and increasing central stellar mass. We investigated the possibility of future observations using the ALMA and mid-infrared high-dispersion spectrographs (e.g., SPICA). We have proposed the water line observations using ALMA, and partial data were delivered.

Possibility to locate the position of the H2O snowline in protoplanetary disks through spectroscopic observations

254th National Meeting (2017)

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