Candidate Water Vapor Lines to Locate the H₂O Snowline Through High-dispersion Spectroscopic Observations. II. The Case of a Herbig Ae Star

Observationally measuring the location of the H₂O snowline is crucial for understanding planetesimal and planet formation processes, and the origin of water on Earth. In disks around Herbig Ae stars (T * ~ 10,000 K, M * > 2.5Msun), the position of the H₂O snowline is farther from the central star compared with that around cooler and less massive T Tauri stars. Thus, the H₂O emission line fluxes from the region within the H₂O snowline are expected to be stronger. In this paper, we calculate the chemical composition of a Herbig Ae disk using chemical kinetics. Next, we calculate the H₂O emission line profiles and investigate the properties of candidate water lines across a wide range of wavelengths (from mid-infrared to submillimeter) that can locate the position of the H₂O snowline. Those lines identified have small Einstein A coefficients (~10¯⁶ - 10¯³ s¯¹) and relatively high upper-state energies (~1000 K). The total fluxes tend to increase with decreasing wavelengths. We investigate the possibility of future observations (e.g., ALMA, SPICA/SMI-HRS) locating the position of the H₂O snowline. Since the fluxes of those identified lines from Herbig Ae disks are stronger than those from T Tauri disks, the possibility of a successful detection is expected to increase for a Herbig Ae disk.