Water ice growth around evolved stars

II. Modeling infrared spectra

¹ Astronomical Institute, “Anton Pannekoek”, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
² Department of Physics and Astronomy, University of Missouri Columbia, 300 Physics Building UMC, Columbia MO 65211, USA e-mail: dijkstrac@missouri.edu
³ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany

Received: 16 August 2005
Accepted: 19 December 2005

Abstract

We present combined radiative transfer and H₂O ice formation calculations for the dusty envelopes of oxygen-rich evolved stars. We study the effects of various (circum-)stellar parameters on the spectral energy distribution of these stars, their infrared spectral water ice features at m and in the m region, and the properties of (water ice on) the grains in their envelopes. We also study the ice formation process as a function of stellar evolution for a star with an initial mass of , which is followed during the AGB, post-AGB and planetary nebula (PN) phase. We find that its water ice features probe its evolution. Both crystalline and amorphous water ice form in our models. The 43 and m crystalline water ice features are most prominent during the post-AGB phase, and only modestly or not present during the AGB and PN phase, in agreement with observations. The strength of the 3, 43 and 62 μm water ice features decreases with decreasing initial mass of the star. The total amount of ice predicted (a few percent of the total dust mass) also agrees with observations, but the crystalline ice mass fraction is consistently about two orders of magnitude lower. This is mainly due to efficient amorphization by interstellar UV photons, and leads to weaker 43 and m crystalline water ice features than observed. The intensity of the interstellar UV radiation field strongly influences the strength of these features. We discuss several means to increase the crystalline water ice mass, and hence their strength. The strength of the features increases dramatically when the mass-loss rate over luminosity ratio of the star, , is large in the AGB phase. In case of the post-AGB star HD 161796 we demonstrate that this indeed leads to the correct crystalline ice mass fraction and feature strengths. Also, the formation of clumps in the AGB wind provides high densities to stimulate the formation of (crystalline) ice. For stars with high initial masses, it additionally provides sufficient shielding from interstellar UV radiation to keep ice crystalline during the post-AGB and PN phase. Axisymmetric mass loss on the AGB provides favorable conditions for the formation and preservation of water ice, and crystalline water ice in particular, as well. In contrast we find that post-AGB crystallization of AGB produced amorphous ice is unimportant for increasing the crystalline water ice mass around  stars.