The circumstellar dust shell of the post-AGB star HD 161796

Abstract

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 2-200 mu m 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 H_2O ice of 70 K, which is higher than thermal equilibrium calculations of pure H_2O 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 H_2O ice mantles the mass loss rate must exceed some 5 *E^-5 M_sun yr^-1. 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 1.2 kpc (Skinner et al. \cite{skinner_HD161796}) and adopting an outflow velocity of 15 km s^-1 (Likkel et al. \cite{likkel_co_pagb}) an AGB mass loss rate of (5.1 x 10^-4 M_sun yr^-1) is found, which lasted 900 years and ended 430 years ago. During this phase a total of 0.46 M_sun was expelled. The mass loss rate was high enough to account for the presence of the H_2O ice. Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, The Netherlands and the UK) and with the participation of ISAS and NASA.