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We report the multi-epoch detections of the water ice in 2.8-4.2 micronspectra of two Herbig Ae stars, PDS 144N (A2 IVe) and PDS 453 (F2 Ve), whichhave an edge-on circumstellar disk. The detected water ice absorption is foundto originate from their protoplanetary disks. The spectra show a relativelyshallow absorption of water ice around 3.1 micron for both objects. The opticaldepths of the water ice absorption are ~0.1 and ~0.2 for PDS 144N and PDS 453,respectively. Compared to the water ice previously detected in low-mass youngstellar objects with an edge-on disk with a similar inclination angle, theseoptical depths are significantly lower. It suggests that stronger UV radiationfrom the central stars effectively decreases the water ice abundance around theHerbig Ae stars through photodesorption. The water ice absorption in PDS 453shows a possible variation of the feature among the six observing epochs. Thisvariation could be due to a change of absorption materials passing through ourline-of-sight to the central star.

Multi-Epoch Detections of Water Ice Absorption in Edge-on Disks around Herbig Ae Stars: PDS 144N and PDS 453