Dust formation around AGB and SAGB stars: a trend with metallicity?

Abstract

We calculate the dust formed around asymptotic giant branch (AGB) and super-AGB stars of metallicity Z = 0.008 by following the evolution of models with masses in the range 1 M M 8 M through the thermal pulses phase, assuming that dust forms via condensation of molecules within a wind expanding isotropically from the stellar surface. We find that, because of the strong hot bottom burning (HBB) experienced, high-mass models produce silicates, whereas lower mass objects are predicted to be surrounded by carbonaceous grains; the transition between the two regimes occurs at a threshold mass of 3.5 M . These findings are consistent with the results presented in a previous investigation, for Z = 0.001. However, in the present higher metallicity case, the production of silicates in the more massive stars continues for the whole AGB phase, because the HBB experienced is softer at Z = 0.008 than at Z = 0.001; thus, the oxygen in the envelope, essential for the formation of water molecules, is never consumed completely. The total amount of dust formed for a given mass experiencing HBB increases with metallicity, because of the higher abundance of silicon, and the softer HBB, both factors favouring a higher rate of silicates production. This behaviour is not found in low-mass stars, because the carbon enrichment of the stellar surface layers, due to repeated third dredge-up episodes, is almost independent of the metallicity. Regarding cosmic dust enrichment by intermediate-mass stars, we find that the cosmic yield at Z = 0.008 is a factor of 5 larger than at Z = 0.001. In the lower metallicity case carbon dust dominates after 300 Myr, but at Z = 0.008 the dust mass is dominated by silicates at all times, with a prompt enrichment occurring after 40 Myr, associated with the evolution of stars with masses M 7.5-8 M . These conclusions are partly dependent on the assumptions concerning the two important macrophysics inputs needed to describe the AGB phase, and still unknown from first principles: the treatment of convection, which determines the extent of the HBB experienced and of the third dredge-up following each thermal pulse, and mass-loss, essential in fixing the time-scale on which the stellar envelope is lost from the star.