JpGU-AGU Joint Meeting 2017

Presentation information

[EJ] Poster

P (Space and Planetary Sciences) » P-PS Planetary Sciences

[P-PS04] [EJ] New developments of planetary sciences with ALMA

Wed. May 24, 2017 5:15 PM - 6:30 PM Poster Hall (International Exhibition Hall HALL7)

[PPS04-P02] Possibility to locate the position of the H2O snowline in protoplanetary disks using high-dispersion spectroscopic observations with ALMA

*Shota Notsu1, Hideko Nomura2, Mitsuhiko Honda3, Tomoya Hirota4, Eiji Akiyama4, Catherine Walsh5, T. J. Millar6 (1.Department of Astronomy, Graduate School of Science, Kyoto University, 2.Department of Earth and Planetary Science, Tokyo Institute of Technology, 3.Department of Physics, School of Medicine, Kurume University, 4.National Astronomical Observatory of Japan, 5.School of Physics and Astronomy, University of Leeds, UK, 6.Astrophysics Research Centre, School of Mathematics and Physics, Queen’s University Belfast, UK)

Keywords:H2O snowline, Protoplanetary disks, Chemical reactions, High-dispersion spectroscopic observations, Molecular emission lines, Planet formation

Inside the H2O snowline of protoplanetary disks, water evaporates from the dust-grain surface into the gas phase, whereas it is frozen out onto the dust in the cold region beyond the snowline. H2O ice enhances the solid material in the cold outer part of a disk, which promotes the formation of gas-giant planet cores. We can regard the H2O snowline as the surface that divides the regions between rocky and gaseous giant planet formation (e.g., Hayashi et al. 1981, 1985). Observationally measuring the location of the H2O snowline is crucial for understanding the planetesimal and planet formation processes, and the origin of water on Earth.

The H2O snowline in the disk midplane around a solar mass T Tauri star is thought to exist at only a few au from the central star. Therefore, the required spatial resolution to directly locate the H2O snowline is on the order of 10 mas (milliarcsecond) around nearby disks (∼100-200 pc) , which remains challenging for current facilities. The velocity profiles of emission lines from protoplanetary disks are usually affected by Doppler shift due to Keplerian rotation and thermal broadening. Therefore, the velocity profiles are sensitive to the radial distribution of the line-emitting regions.

In this study (Notsu et al. 2016, 2017), we propose the method to locate the position of the H2O snowline in protoplanetary disks through the observations of H2O line profiles, on the basis of our calculations. First, we calculated the chemical composition of a T Tauri disk (Tstar~4,000K, Mstar~0.5Msun) and a Herbig Ae disk (Tstar~10,000K, Mstar~2.5Msun) using chemical kinetics. We confirmed that the abundance of H2O is high not only in the inner region of H2O snowline near the equatorial plane but also in the hot surface layer and photodesorption region of the outer disk.

Next, we calculated the H2O emission line profiles, and investigate the properties of candidate water lines across a wide range of wavelengths (from mid-infrared to sub-millimeter) that can locate the position of the H2O snowline. Those identified lines have small Einstein A coefficients (~10−6-10−3 s−1) and relatively high upper state energies (~1000K). The total fluxes tend to increase with decreasing wavelengths. In disks around Herbig Ae stars, the position of the H2O snowline is further from the central star compared with that around cooler, and less luminous T Tauri stars. Thus, the H2O emission line fluxes from the region within the H2O snowline are stronger for the Herbig Ae disks.

In this presentation, we introduce results of our calculations explained above, and discuss the possibility of observations with ALMA to locate the position of the H2O snowline.
In addition, recently we have calculated the H2O line profiles in the wavelength region of ALMA band 5. We wll also introduce those results.

Reference; Notsu, S., et al. 2016, ApJ, 827, 113
Notsu, S., et al. 2017, ApJ, 836, 118