Dust Production from Sub-Solar to Super-Solar Metallicity in Thermally Pulsing Asymptotic Giant Branch Stars

摘要

We discuss the dust chemistry and growth in the circumstellar envelopes (CSEs) of Thermally Pulsing Asymptotic Giant Branch (TP-AGB) star models, computed with the COLIBRI code, at varying initial mass and metallicity (Z = 0.001, 0.008, 0.02,0.04, 0.06). A relevant result of our analysis deals with silicate production in M stars. We show that, in order to reproduce the observed trend between terminal velocities and mass-loss rates in Galactic M giants, one has to significantly reduce the efficiency of chemisputtering by H_2 molecules, usually considered the most effective dust destruction mechanism. This conclusion is in agreement with the most recent laboratory results, which show that silicates may condense already at T_(cond) ?1400 K,rather than only at T_(cond) -1000 K, as obtained by models that include chemisputtering. From analysis of the total dust ejecta, we find that the dust-to-gas ratios of the total ejecta from intermediate-mass stars are much less dependent on metallicity than usually assumed. In a broader context, our results are suitable for studying the dust enrichment of the interstellar medium provided by TP-AGB stars in both nearby and high-redshift galaxies.