Detection of Water Ice in Edge‐on Protoplanetary Disks: HK Tauri B and HV Tauri C

Abstract

We report the first detection of 3 μm water ice absorption in a protoplanetary disk. Low-resolution spectroscopy with Subaru IRCS was carried out for two T Tauri stars with an edge-on disk, HK Tau B and HV Tau C. A 3 μm deep water ice absorption (τ ≥ 1) toward both objects was detected. Contribution by foreground cloud material to the ice absorption should be small, since AV ≤ 3 toward HK Tau A and HV Tau A, HV Tau B. Although HV Tau C is reported to have a small amount of envelope material, its mass is insufficient to produce the large optical depth of the detected water ice. In addition, HK Tau B does not have any significant envelope mass. Therefore, the water ice exists inside the disks of these objects. The optical depth profile of the water ice absorption in the protoplanetary disks does not show any significant difference from those of protostellar sources. The water ice optical depth for HV Tau C showed a large variation (Δτ = 0.59) in two observing epochs separated by 2.32 yr. Assuming the ice absorption comes from a spherical cloud at 100 AU and Keplerian rotation, it suggests that a Pluto mass gas and dust cloud with a size of 1.4 AU may have passed by the line of sight. The lack of variability of the continuum level of HV Tau C at the two epochs implies that the dust grains with an icy mantle are segregated from the grains without an icy mantle.