C Dijkstra (University of Missouri-Columbia, USA), C Dominik, A de Koter (Astronomical Institute, ``Anton Pannekoek'', University of Amsterdam, The Netherlands), J Bouwman (Max-Planck-Institut für Astronomie, Heidelberg, Germany)
We present combined radiative transfer and H2O ice formation calculations for the dusty winds of oxygen-rich evolved stars. We study the effects of various stellar and circum-stellar parameters on the spectral energy distribution, the infrared spectral features of water ice at 3 um and in the 30-100 um region, and the properties of (the water ice on) the grains in the envelope. Moreover, we study the ice formation process as a function of stellar evolution for stars with initial masses of 3 and 5 Msun. We follow these stars during the AGB, post-AGB and planetary nebula phase. We find that the 3, 43 and 62 um water ice features in the spectra of evolving low mass stars can be used to probe their wind properties, and, consequently, their evolution. The most prominent 43 and 62 um water ice features are seen in the post-AGB phase, while only modest or no features are seen during the AGB and planetary nebula phase. The mass loss rates required in the AGB phase to explain the strength of observed features must significantly higher than currently adopted. We discuss several additional means to increase the crystalline water ice mass, and thus the strength of the 43 and 62 um crystalline water ice features. Here, the crystallization of amorphous ice during post-AGB evolution is found to be unimportant for stars of 5 Msun and less. 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 may also provide sufficient shielding from interstellar UV radiation to keep the ice crystalline during the post-AGB and planetary nebula phases. Asymmetric mass loss on the AGB may provide favorable conditions for the formation and preservation of water ice, and crystalline water ice in particular. Finally, we find that the intensity of the interstellar UV radiation field strongly influences the strength of the far infrared crystalline water ice features.
Bulletin of the American Astronomical Society, 37 #2
© 2005. The American Astronomical Soceity.