Water lines and multiple ring and gap structures of the protoplanetary disk around HD 163296

Observationally locating the position of the H2O snowline (=the sublimation front of water molecules) in protoplanetary disks is crucial because it will constrain the dust-grain evolution and planet formation, and the origin of water on terrestrial planets. Water lines from disks have been detected through recent space infrared spectroscopic observations, such as Spitzer and Herschel. However, these lines mainly trace the disk surface and the cold water reservoir outside the H2O snowline. In our previous papers (Notsu et al. 2016&2017&2018, ApJ), on the basis of our calculations of disk chemical structures and water line profiles, we proposed how to identify the position of the H2O snowline in the disk midplane by analyzing the Keplerian profiles of water lines, which can be obtained by high-dispersion spectroscopic observations across a wide range of wavelengths. We concluded that water lines that have small Einstein A coefficients and relatively high excitation energies trace the hot water gas inside the H2O snowline. In ALMA Cycle 3, we conducted the high-spatial resolution continuum and water line observations at Band 7 for the disk around the Herbig Ae star HD163296 (Notsu et al. 2019, ApJ, 875, 96). The targeted lines are considered to be the candidate submillimeter water lines to locate the position of the H2O snowline, on the basis of our model calculations. We compared the upper limit fluxes with the values calculated by our model calculations with dust emission included, and we constrained the line emitting region (the position of the H2O snowline) and the dust opacities from the observations. We conclude that, if the outer edge of the region with a high water abundance and the position of the water snowline are both beyond 8 au, then the millimeter dust opacity will have a value larger than 2.0 cm^2 g^-1. In addition, the position of the water snowline must lie inside 20 au if the millimeter dust opacity is 2.0 cm^2 g^-1. Moreover, we also reported multiple ring and gap patterns in the Band 7 dust continuum emission with around 15 au resolution. The positions of bright rings and dust depleted dark gaps are consistent with those indicated by the previous ALMA observations (e.g., Isella et al. 2016, 2018). Future observations of the dust continuum emission at higher angular resolution and water lines with longer observation time are required to clarify the detailed structures and the position of the H2O snowline in the disk midplane