Candidate Water Vapor Lines to Locate the H 2 O Snowline Through High-dispersion Spectroscopic Observations. II. The Case of a Herbig Ae Star

Abstract

Observationally measuring the location of the H 2 O 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 *  ≳ 2.5 M ⊙ ), the position of the H 2 O snowline is farther from the central star compared with that around cooler and less massive T Tauri stars. Thus, the H 2 O emission line fluxes from the region within the H 2 O 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 H 2 O 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 H 2 O 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 H 2 O 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.