Mineral formation in stellar winds

Abstract

Abstract. S stars are transition objects at the top of the AGB between M-and C-stars. The third dredge-up in thermally pulsating AGB-stars transports freshly synthesised carbon from the He burning zone into the stellar atmosphere. If the carbon and oxygen abundance are about equal the star is classified as being of spectral type S. Stars at the top of the AGB are subject to large mass-loss caused by a strong stellar wind. At some distance from the star the temperature of the gas drops below the condensation temperature of some mineral compounds and soot. This results in optically thick dust shells which are sources of an intense IR-radiation from warm dust. The composition of the dust formed in the stellar outflow critically depends on the C/O-ratio. For the standard element mixture characteristic for main sequence stars one has C/O < 1 (M-stars) and the O not bound in CO forms a mineral assemblage which is dominated by Mg-Fe-silicates and metallic Fe. For C/O > 1 (C-stars) the dust mixture is dominated by solid carbon and some SiC. The element mixture of S stars is characterised by the non-availability of sufficient quantities of O or C to form the standard condensates. We have investigated the condensation processes for the peculiar element mixture at the M-S-C transition on the AGB. From thermodynamic equilibrium calculations we find that the most likely solids to be formed are solid FeSi, metallic iron and small quantities of forsterite and SiC. Nucleation of dust may be triggered by TiC, ZrC or TiO2. For these substances, non-equilibrium dust condensation in the outflow is calculated for a simple stationary wind model for a sample of C/O-ratios. The results of our calculation show that iron and FeSi dust condensates in the circumstellar shells of S stars.

Low-temperature gas opacity

“…We may say that these two C/O values bracket the transition region between the O-dominated and the C-dominated chemistry. As discussed by Ferrarotti & Gail (2002) the abrupt changes in the chemical equilibria at C/O ≈ 0.93 and C/O ≈ 1.0 respectively correspond to the critical values of the carbon abundance…”

“…C, O, and to a less extent Si. Following Ferrarotti & Gail (2002) for temperatures T < ∼ 1500 K, at which dust is expected to condensate, one must also consider the contribution of another strongly-bound molecule, SiS (E B (SiS) = 6.46 eV), so that the first critical carbon abundance should be redefined as ε crit,1 C = ε O − ε Si + ε S . Since this study deals with the gas chemistry for log(T ) ≥ 3.2 (i.e.…”

Analysis of ISO SWS01 spectra of S stars

“…However our identification results seem to show that some S stars have CS and HCN absorption features. Ferrarotti & Gail (2002) found that a dramatic change of gas composition occurs in the condensation zone of circumstellar shells with lower temperatures (e.g., T < 1200 K) at a critical carbon abundance of ε C,crit = ε O −ε Si +ε S (=0.975ε O ;…”

“…here ε x means the abundance of x element) (Ferrarotti & Gail 2002). When the carbon abundance is less than ε C,crit , the gas composition is almost dominated by O-rich molecules, while when the carbon abundance is more than ε C,crit the gas composition is almost dominated by C-rich molecules.…”

“…Currently, intrinsic S stars are observed to have CDEs, however extrinsic S stars do not present CDEs (Jorissen et al 1993), which is consistent with the current understanding that extrinsic S stars are not in the AGB phase. Since S stars have the C/O ratio of the transitional range from oxygen-to carbon-dominated molecules and dust materials, their chemical properties in atmospheres and CDEs are different from those of both M and C stars (Ferrarotti & Gail 2002) and the differences should be reflected in molecule and dust features on infrared range. Therefore, it is very necessary to study the infrared spectra of S stars.…”