Water ice growth around evolved stars

Abstract

We present combined radiative transfer and H 2 O 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 3 µm and in the 30−100 µm 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 5 M , 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 62 µm 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 62 µm 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,Ṁ/L, 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 5 M stars.

Iras 17423–1755 (Hen 3–1475) Revisited: An O-Rich High-Mass Post-Asymptotic Giant Branch Star

“…Additionally, the ISO spectrum of IRAS 19343+2926 shows the presence of the water ice bands at 62 and 43 μm (Dijkstra et al 2006), confirming the crystalline nature of the water ice in this star. The presence of these latter features cannot be excluded in the case of IRAS 17423−1755 because of the poor quality of the ISO LWS spectrum (see Figure 2).…”

“…Water ice has been observed in a significant sample of AGB stars-in particular in OH/IR stars-some post-AGB stars and a few PN (Omont et al 1990;Eiroa & Hodapp 1989;Hoogzaad et al 2002;Molster et al 2001). The specific conditions for the formation of water ice in circumstellar envelopes around evolved stars have been discussed and modeled by Dijkstra et al (2003Dijkstra et al ( , 2006, who also studied the trends that can be observed in the 3, 43, and 62 μm water ice features during the stellar evolution from the AGB to the PN phase.…”

“…According to Dijkstra et al (2006) it continues to be a puzzle why some stars form water ice and not others. One clue these authors found might be the fact that the strength of the 3, 43, and 62 μm water-ice features increases with the increasing initial mass of the star.…”

“…The models developed by Dijkstra et al (2006) show that the shape of the spectral features is very sensitive to the type of water ice aggregate formed. The 3.1 μm crystalline water ice absorption feature is characterized by a sharp core with two unequally strong shoulders.…”

“…Additionally water-ice features at 3.1, 43, and 62 μm have been observed in heavily obscured and extremely bipolar sources such as the post-AGB star IRAS 19343+2926 (or M1−92; see, e.g., Dijkstra et al 2006, and references therein). Gauba & Parthasarathy (2004) studied the ISO spectra of seven hot post-AGB stars including IRAS 17423−1755.…”