2002A&A...389..547H

We have modeled the complete optical to millimeter spectrum of the Post-Asymptotic Giant Branch (Post-AGB) star HD 161796 and its circumstellar dust shell. A full 2-200µm spectrum taken with the Infrared Space Observatory was used to constrain the dust properties. A good fit is achieved using only 4 dust components: amorphous silicates, the crystalline silicates forsterite and enstatite, and crystalline water ice, contributing respectively about 63, 4, 6 and 27% to the total dust mass. The different dust species were assumed to be co-spatial but distinct, resulting in different temperatures for the different grain populations. We find a temperature for the crystalline H₂O ice of 70K, which is higher than thermal equilibrium calculations of pure H₂O ice would give. This implies that the ice must be formed as a mantle on top of an (amorphous) silicate core. In order to form H₂O ice mantles the mass loss rate must exceed some 5x10^–5M_☉/yr. With a water-ice fraction of 27% a lower limit for the gas to dust mass ratio of 270 is found. At a distance of 1.2kpc (Skinner et al., 1994ApJ...423L.135S) and adopting an outflow velocity of 15km/s (Likkel et al., 1991A&A...246..153L) an AGB mass loss rate of (5.1x10^–4M_☉/yr) is found, which lasted 900 years and ended 430 years ago. During this phase a total of 0.46M_☉ was expelled. The mass loss rate was high enough to account for the presence of the H₂O ice.