Water and 13C17O lines, and multiple ring and gap structures of the protoplanetary disk around HD 163296 observed by ALMA

Observationally locating the position of the H2O snowline (e.g., Hayashi et al. 1981, 1985) in protoplanetary disks is crucial for understanding the dust evolution 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 works (Notsu et al. 2016, ApJ, 827, 113; 2017, ApJ, 836, 118; 2018, ApJ, 855, 62), we calculated the chemical composition of the disks around a T Tauri star and a Herbig Ae star using chemical kinetics and various water line profiles. 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. Since the fluxes of these lines from Herbig Ae disks are larger than those from T Tauri disks, the possibility of a successful detection is expected to increase for a Herbig Ae disk. There are several best candidate water lines that trace the position of the H2O snowline within the coverage of ALMA. Recently, we got the upper limit fluxes of submillimeter ortho-H2(16)O 321 GHz, para-H2(18)O 322 GHz, and HDO 335 GHz lines from the disk around the Herbig Ae star HD 163296, using ALMA (Notsu et al. 2019, ApJ, 875, 96). These water lines are considered to be the candidate water lines to locate the position of the H2O snowline, based on our model calculations. We compared the upper limit fluxes with the values calculated by our model calculations with dust emission, and we constrained the line emitting region and the dust opacity from the observations. We also detected multiple ring and gap patterns in the 0.9 mm (ALMA Band 7) dust continuum emission with 15 au spatial resolution, whose positions are consistent with those indicated by other observations (e.g., Isella et al. 2018).  Future observations of the submillimeter water lines with longer observation time are required to clarify the position of the H2O snowline in the disk midplane. In addition, we also detected the rarest stable CO isotopologue, 13C17O, in a disk for the first time (Booth et al. 2019, ApJL, 882, L31). We compared our observation with the existing detections of other CO isotoplogues in the HD163296 disk. We found that this line is optically thin within the CO snowline and will be thus a robust tracer of the bulk disk CO gas mass. We showed that this disk will be 2-6 times more massive than previously estimates.