Infrared Spectra of Highly Reddened Stars: a Search for Interstellar Ice Grains

Abstract

Infrared spectra extending to 3.5 ~ have been obtained of the highly reddened supergiants \TI[ Cyg No. 12, CIT 11, and HD 183143. No absorption band at 3.07 ~ stronger than 10 percent of the expected absorption of ice grains can be fitted to the spectra of the first two objects, and none greater than 50 percent to the third. Theoretical ca]culations have been made of the absorption cross-sections for pure-ice grains and for composite grains consisting of ice mantles surrounding a graphite core. New laboratory spectra of the position, strength, and shape of the ice band confirm that these calculations are valid for interste~1ar temperatures. It is concluded that the interstellar grains contain very little ice