Modeling Studies II. The Case of the Herbig Ae Star

Abstract

Observationally locating the position of the H$_{2}$O snowline in protoplanetary disks is important to understand the processes of planetesimal and planet formation, and the origin of water on terrestrial planets including the Earth. In our studies, first we calculated chemical structures of the disk using the self-consistent physical models of a typical Herbig Ae disk. Next, on the basis of our calculations of disk chemical structures and water line profiles, we proposed how to identify the H$_{2}$O snowline positions directly by analyzing the Keplerian water line profiles which can be obtained by high-dispersion spectroscopic observations across a wide range of wavelengths (from mid-infrared to sub-millimeter wavelengths). We selected candidate water lines to locate the H$_{2}$O snowline based on specific criteria. We concluded that water lines with small Einstein A coefficients (A$_{\mathrm {ul}} = 10^{-6}{\sim } 10^{-3}$ s$^{-1}$) and relatively high upper state energies (E$_{\mathrm {up}}$ $\sim$ 1000 K) trace the hot water vapor within the ${\mathrm {H_2O}}$ snowline, and can locate the H$_{2}$O snowline positions. In these candidate water lines, the contribution of the optically thick hot midplane within the H$_{2}$O snowline is large compared with that of the outer optically thin surface layer. This is because the line intensities from the optically thin region are proportional to the Einstein A coefficient. In addition, the contribution of the cold water reservoir outside the H$_{2}$O snowline is also small, since lines with high excitation energies are not emitted from the regions at a low temperature. The H$_{2}$O snowline positions of a Herbig Ae disk exists at a larger radius compared with that around cooler and less massive T Tauri stars. Moreover, the H$_{2}$O snowline position migrates closer to the star as the disk becomes older and mass accretion rate to the central star becomes smaller. Thus, observing the candidate water lines and locating the H$_{2}$O snowline positions, in Herbig Ae disks and younger T Tauri stars, is expected to be easier. We investigate the possibility of future observations (e.g., ALMA, SPICA/SMI-HRS) to locate the ${\mathrm {H_2O}}$ snowline position. Most contents of this chapter is based on our refereed paper that has been published (Notsu et al. 2017, ApJ, 836, 118).