Dust‐Grain Processing in Circumbinary Discs Around Binary Post‐AGB Stars Available to Purchase

The number of binary post‐AGB stars known in the galaxy is significant, yet their evolutionary status is far from understood. The orbital elements from our radial velocity monitoring program indicate that these objects must have undergone severe binary interaction, when the primary was at giant dimension. Evidence is growing that all these evolved binaries are surrounded by circumbinary compact Keplerian dusty discs. We present the first results of our SED‐modelling and detailed study of the mineralogy using the rich Spitzer‐IRS and TIMMI2 spectra. In all sampled stars, the dust is oxygen rich with a variable but, on average, high degree of crystallinity. Features of the crystalline olivine and pyroxene (forsterite and enstatite) dominate the spectra. Both cool and hot crystalline dust grains must be present to produce the observed dust emission features. Our 2D SED‐modelling shows that the presence of a component of large grains is needed to account for the 850 micron flux. This, together with the high crystallinity, is evidence for efficient dust processing in these circumstellar discs. Since the binaries span a significant range in initial metallicity, orbital separation and mass functions we conclude that the circumstellar dust around a significant population of binaries is very different from dust formed and processed around single stars.