Dust from asymptotic giant branch stars: relevant factors and modelling uncertainties

The dust formation process in the winds of asymptotic giant branch stars is discussed, based on full evolutionary models of stars with mass in the range 1 <= M <= 8 M-circle dot and metallicities 0.001 < Z < 0.008. Dust grains are assumed to form in an isotropically expanding wind, by growth of pre-existing seed nuclei. Convection, for what concerns the treatment of convective borders and the efficiency of the schematization adopted, turns out to be the physical ingredient used to calculate the evolutionary sequences with the highest impact on the results obtained. Low-mass stars with M <= 3 M-circle dot produce carbon-type dust with also traces of silicon carbide. The mass of solid carbon formed, fairly independently of metallicity, ranges from a few 10(-4) M-circle dot, for stars of initial mass 1-1.5 M-circle dot, to similar to 10(-2) M-circle dot, for M-circle dot 2-2.5 M-circle dot; the size of dust particles is in the range 0.1 <= a(C) <= 0.2 mu m. On the contrary, the production of silicon carbide (SiC) depends on metallicity. For 10(-3) <= Z <= 8 x 10(-3) the size of SiC grains varies in the range 0.05 < a(SiC) < 0.1 mu m, while the mass of SiC formed is 10(-5) < M-SiC < 10(-3) M-circle dot. Models of higher mass experience hot bottom burning, which prevents the formation of carbon stars, and favours the formation of silicates and corundum. In this case the results scale with metallicity, owing to the larger silicon and aluminium contained in higher Z models. At Z = 8x10(-3) we find that themostmassive stars produce dust masses m(d) similar to 0.01 M-circle dot, whereas models of smaller mass produce a dust mass 10 times smaller. The main component of dust is silicates, although corundum is also formed, in not negligible quantities (similar to 10(-20) per cent).