Gas and dust from extremely metal-poor AGB stars
  We study the evolution of extremely metal-poor AGB stars, with metallicities
down to [Fe/H]=-5, to understand the main evolutionary properties, the
efficiency of the processes able to alter their surface chemical composition
and to determine the gas and dust yields. We calculate two sets of evolutionary
sequences of stars in the 1-7.5Msun mass range, evolved from the pre-main
sequence to the end of the AGB phase. To explore the extremely metal-poor
chemistries we adopted the metallicities Z=3x10^{-5} and Z=3x10^{-7} which
correspond, respectively to [Fe/H]=-3 and [Fe/H]=-5. The results from stellar
evolution modelling are used to calculate the yields of the individual chemical
species. We also modelled dust formation in the wind, to determine the dust
produced by these objects. The evolution of AGB stars in the extremely
metal-poor domain explored here proves tremendously sensitive to the initial
mass of the star. M<2Msun stars experience several third dredge-up events,
which favour the gradual surface enrichment of C12 and the formation of
significant quantities of carbonaceous dust, of the order of 0.01Msun. The C13
and nitrogen yiel are found to be significantly smaller than in previous
explorations of low-mass, metal-poor AGB stars, owing to the weaker proton
ingestion episodes experienced during the initial AGB phases. M>5Msun stars
experience hot bottom burning and their surface chemistry reflects the
equilibria of a very advanced proton-capture nucleosynthesis; little dust
production takes place in their wind. Intermediate mass stars experience both
third dredge-up and hot bottom burning: they prove efficient producers of
nitrogen, which is formed by proton captures on C12 nuclei of primary origin
dredged-up from the internal regions.
