Near-infrared observations of water-ice in OH/IR stars

A search for the near-infrared water-ice absorption band was made in a number of very red OH/IR stars which are known to exhibit the 10 μm silicate absorption. As a by-product, accurate positions of these highly reddened objects are obtained. We derived a dust mass loss rate for each object by modelling the spectral energy distribution and the gas mass loss rate by solving the equation of motion for the dust drag wind. The derived mass loss rates show a strong correlation with the silicate optical depth as well as that of the water-ice. The stars have a high mass loss rate (${>} 10^{-4}$ $M_{\odot}$ yr-1) with an average gas-to-dust mass ratio of 110. In objects which show the 3.1 μm water-ice absorption, the near-IR slope is much steeper than those with no water-ice. Comparison between our calculated mass loss rates and those derived from OH and CO observations indicates that these stars have recently increased their mass loss rates.