Ice grains play an important role on star and planet formation and related matters. Therefore, to reveal ice grain properties within a star and planet forming region is critical to understand the evolution of icy grains. Among various ice composition, water ice is usually dominant and have several vibration bands in mid- to far-infrared. Especially, 3.1 µm OH vibration mode is strong, and provide a good tracer of water ice. Combined with polarimetry information, Tazaki et al. 2021 showed that the grain size and abundance could be constrained. Here we propose to make L-band spectro-polarimetric observations of protostars to constrain water ice grain size and abundance in the envelope of protostars. Only Subaru/IRCS have unique thermal infrared (TIR) polarization capabilities among 8-10m class telescopes, thus the proposed observations make use of the unique capability of Subaru telescope. Recently, the depletion of water ice abundance is suggested in the various environment such as surface of envelope and protoplanetary disks probably due to non-thermal desorption of water molecule from the grains, thus our proposed observation will provide fundamental observational evidence whether such phenomenon is common or not.
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