Super and massive AGB stars – III. Nucleosynthesis in metal-poor and very metal-poor stars – Z = 0.001 and 0.0001

Abstract

We present a new grid of stellar models and nucleosynthetic yields for\nsuper-AGB stars with metallicities Z=0.001 and 0.0001, applicable for use\nwithin galactic chemical evolution models. Contrary to more metal rich stars\nwhere hot bottom burning is the main driver of the surface composition, in\nthese lower metallicity models the effect of third dredge-up and corrosive\nsecond dredge-up also have a strong impact on the yields. These metal-poor and\nvery metal-poor super-AGB stars create large amounts of He4, C13 and N14, as\nwell as the heavy magnesium isotopes Mg25 and Mg26. There is a transition in\nyield trends at metallicity Z approximately 0.001, below which we find positive\nyields of C12, O16, N15, Al27 and Si28, which is not the case for higher\nmetallicities. We explore the large uncertainties derived from wind\nprescriptions in super-AGB stars, finding approximately 2 orders of magnitude\ndifference in yields of Ne22, Na23, Mg24,25,26, Al27 and our s-process proxy\nisotope g. We find inclusion of variable composition low temperature molecular\nopacities is only critical for super-AGB stars of metallicities below Z\napproximately 0.001. We analyze our results, and those in the literature, to\naddress the question: Are super-AGB stars the polluters responsible for extreme\npopulation in the globular cluster NGC 2808? Our results, as well as those from\nprevious studies, seem unable to satisfactorily match the extreme population in\nthis globular cluster.\n