The dust condensation sequence in red super-giant stars

Context. Red super-giant (RSG) stars exhibit significant mass loss through a slow and dense wind. They are often considered to be the more massive counter-parts of Asymptotic Giant Branch (AGB) stars. While the AGB mass-loss is linked to their strong pulsations, the RSG are often only weakly variable. This raises the question whether their wind-driving mechanism and the dust composition in the wind are the same. Aims. To study the conditions at the base of the wind, by determining the dust composition in a sample of RSG. The dust composition is thought to be sensitive to the density, temperature and acceleration at the base of the wind. To compare the derived composition with the composition found in AGB star winds. Methods. We compile a sample of 27 RSG infrared spectra (ISO-SWS) and supplement these with photometric measurements to obtain the full spectral energy distribution (SED). These data are modelled using a dust radiative transfer code, taking into account the optical properties of relevant candidate materials. The results are scrutinised for correlations in terms of mass-loss rate, density at the inner edge of the dust shell and stellar parameters. Results. We find (1) strong correlations between dust composition, mass-loss rate and stellar luminosity, roughly in agreement with the theoretical dust condensation sequence, (2) the need for a continuous (near-)IR dust opacity and tentatively propose amorphous carbon, and (3) significant differences with AGB star winds: presence of PAHs, absence of ’the’ 13 μm band, and a lack of strong water bands. Conclusions. Dust condensation in RSG is found to experience a similar freeze-out process as in AGB stars. Together with the positive effect of the stellar luminosity on the mass-loss rate, this suggests that radiation pressure on dust grains is an important ingredient in the driving mechanism. Still, differences with AGB stars are manifold and thus the winds of RSG deserve separate studies.