WATER ICE IN HIGH MASS-LOSS RATE OH/IR STARS

Abstract

We investigate water-ice features in spectral energy distributions (SEDs) of high mass-loss rate OH/IR stars. We use a radiative transfer code which can consider multiple components of dust shells to make model calculations for various dust species including water ice in the OH/IR stars. We find that the model SEDs are sensitively dependent on the location of the water-ice dust shell. For two sample stars (OH 127.8+0.0 and OH 26.5+0.6), we compare the detailed model results with the infrared observational data including the spectral data from the Infrared Space Observatory (ISO). For the two sample stars, we reproduce the crystalline water-ice features (absorption at 3.1 μm and 11.5 μm; emission at 44 and 62 μm) observed by ISO using a separate component of the water-ice dust shell that condensed at about 84-87 K (r ~ 1500-1800 AU) as well as the silicate dust shell that condensed at about 1000 K (r ~ 19-25 AU). For a sample of 1533 OH/IR stars, we present infrared two-color diagrams (2CDs) using the Infrared Astronomical Satellite and AKARI data compared with theoretical model results. We find that the theoretical models clearly show the effects of the crystalline water-ice features (absorption at 11.5 μm and emission at 62 μm) on the 2CDs.