AGB dust and gas ejecta in extremely metal-poor environments
  We present asymptotic giant branch (AGB) models of metallicity $Z=10^{-4}$
and $Z=3\times 10^{-4}$, with the aim of understanding how the gas enrichment
and the dust production change in very metal-poor environments and to assess
the general contribution of AGB stars to the cosmic dust yield. The stellar
yields and the dust produced are determined by the change in the surface
chemical composition, with a transition occurring at $\sim 2.5~M_{\odot}$.
Stars of mass $M < 2.5~M_{\odot}$ reach the carbon stage and produce carbon
dust, whereas their higher mass counterparts produce mainly silicates and
alumina dust; in both cases the amount of dust manufactured decreases towards
lower metallicities. The $Z=10^{-4}$ models show a complex and interesting
behaviour on this side, because the efficient destruction of the surface oxygen
favours the achievement of the C-star stage, independently of the initial mass.
The present results might indicate that the contribution from this class of
stars to the overall dust enrichment in metal-poor environments is negligible
at redshifts $z>5$.
