  We report the multi-epoch detections of the water ice in 2.8-4.2 micron
spectra of two Herbig Ae stars, PDS 144N (A2 IVe) and PDS 453 (F2 Ve), which
have an edge-on circumstellar disk. The detected water ice absorption is found
to originate from their protoplanetary disks. The spectra show a relatively
shallow absorption of water ice around 3.1 micron for both objects. The optical
depths 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 young
stellar objects with an edge-on disk with a similar inclination angle, these
optical depths are significantly lower. It suggests that stronger UV radiation
from the central stars effectively decreases the water ice abundance around the
Herbig Ae stars through photodesorption. The water ice absorption in PDS 453
shows a possible variation of the feature among the six observing epochs. This
variation could be due to a change of absorption materials passing through our
line-of-sight to the central star.
