Super and massive AGB stars - II. Nucleosynthesis and yields - Z = 0.02, 0.008 and 0.004

We have computed detailed evolution and nucleosynthesis models for super and massive asymptotic giant branch (AGB) stars over the mass range 6.5-9.0 M_⊙ in divisions of 0.5 M_⊙ with metallicities Z = 0.02, 0.008 and 0.004. These calculations, in which we find third dredge-up and hot bottom burning, fill the gap between existing low- and intermediate-mass AGB star models and high-mass star models that become supernovae. For the considered metallicities, the composition of the yields is largely dominated by the thermodynamic conditions at the base of the convective envelope rather than by the pollution arising from third dredge-up. We investigate the effects of various uncertainties, related to the mass-loss rate, mixing length parameter, and the treatment of evolution after the envelope instability that develops near the end of the (super)AGB phase. Varying these parameters alters the yields mainly because of their impact on the amount of third dredge-up enrichment, and to a lesser extent on the hot bottom burning conditions. Our models produce significant amounts of ⁴He, ⁷Li (depending on the mass-loss formulation) ¹³C, ¹⁴N, ¹⁷O, ²³Na, ²⁵Mg, as well the radioactive isotope ²⁶Al in agreement with previous investigation. In addition, our results show enrichment of ²²Ne, ²⁶Mg and ⁶⁰Fe, as well as a substantial increase in our proxy neutron capture species representing all species heavier than iron. These stars may provide important contributions to the Galaxy's inventory of the heavier Mg isotopes, ¹⁴N, ⁷Li and ²⁷Al.