Dust formation in the winds of AGBs: the contribution at low metallicities

We present new models for the evolution of stars with mass in the range 1M(circle dot) <= M <= 7.5M(circle dot), followed from the pre-main sequence through the asymptotic giant branch (AGB) phase, until most of their envelope is lost via stellar winds. The metallicity adopted is Z = 3x10(-4) ( which, with an alpha-enhancement of + 0.4, corresponds to [Fe/H] = - 2). Dust formation is described by following the growth of dust grains of various types as the wind expands from the stellar surface. Models with mass M >= 3 M-circle dot experience hot bottom burning, thus maintaining the surface C/O below unity. Unlike higher Z models, the scarcity of silicon available in the envelope prevents the formation of silicates in meaningful quantities, sufficient to trigger the acceleration of the wind via radiation pressure on the dust grains formed. No silicate formation occurs below a threshold metallicity of Z = 10(-3). Low-mass stars, with M < 2.5 M-circle dot become carbon stars, forming solid carbon-dust in their surroundings. The total dust mass formed depends on the uncertain extent of the inwards penetration of the convective envelope during the third dredge-up episodes following the thermal pulses. However, provided that a minimum abundance of carbon of X(C) similar to 5 x 10(-3) is reached in the envelope, the results turn out to be fairly independent of the parameters used. Carbon grains have sizes 0.08 < a(C) < 0.12 mu m and the total amount of dust formed (increasing with the mass of the star) is M-C = (2-6) x 10(-4) M-circle dot. Our results imply that AGB stars with Z = 3 x 10(-4) can only contribute to carbon-dust enrichment of the interstellar medium on relatively long time-scales, > 300 Myr, comparable to the evolutionary time of a 3 M-circle dot star. At lower metallicities, the scarcity of silicon available and the presence of hot bottom burning even in M < 2 M-circle dot, prevents the formation of silicate and carbon grains. We extrapolate our conclusion to more metal-poor environments, and deduce that at Z < 10(-4) dust enrichment is mostly due to metal condensation in supernova ejecta.