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 the planetesimal and planet formation processes, and the origin of water on Earth. In disks around Herbig Ae stars ($T_{\ast }\sim$ 10,000K, $M_{\ast }\gtrsim$ 2.5$M_{⨀}$), the position of the H${}_2$O snowline is further 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 sub-millimeter) that can locate the position of the H${}_2$O snowline. Those line identified have small Einstein $A$ coefficients ($\sim 10^{-6}-10^{-3}$ s${}^{-1}$) and relatively high upper state energies ($\sim$ 1000K). The total fluxes tend to increase with decreasing wavelengths. We investigate the possibility of future observations (e.g., ALMA, SPICA/SMI-HRS) to locate 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.

39 pages, 14 figures, and 2 tables are contained in this paper. It was received by The Astrophysical Journal (ApJ) on October 27th, 2016, and was accepted on January 13th, 2017. arXiv admin note: text overlap with arXiv:1606.05828

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