Evolution and nucleosynthesis of extremely metal-poor and metal-free low- And intermediate-mass stars I. Stellar yield tables and the CEMPs

Abstract

Context. The growing body of spectral observations of the extremely metal-poor (EMP) stars in the Galactic Halo provides constraints on theoretical studies of the chemical and stellar evolution of the early Universe. Aims. To calculate yields for EMP stars for use in chemical evolution calculations and to test whether such models can account for some of the recent abundance observations of EMP stars, in particular the highly C-rich EMP (CEMP) halo stars. Methods. We modify an existing 1D stellar structure code to include time-dependent mixing in a diffusion approximation. Using this code and a post-processing nucleosynthesis code we calculate the structural evolution and nucleosynthesis of a grid of models covering the metallicity range: -6.5 ≤ [Fe/H] ≤ -3.0 (plus Z = 0), and mass range: 0.85 ≤ M ≤ 3.0 M ⊙, amounting to 20 stars in total. Results. Many of the models experience violent nuclear burning episodes not seen at higher metallicities. We refer to these events as "Dual Flashes" since they are characterised by nearly simultaneous peaks in both hydrogen and helium burning. These events have been reported by previous studies. Some of the material processed by the Dual Flashes is dredged up causing significant surface pollution with a distinct chemical composition. We have calculated the entire evolution of the Z = 0 and BMP models, from the ZAMS to the end of the TPAGB, including extensive nucleosynthesis. In this paper, the first of a series describing and analysing this large data set, we present the resulting stellar yields. Although subject to many uncertainties these are, as far as we are aware, the only yields currently available in this mass and metallicity range. We also analyse the yields in terms of C and N, comparing them to the observed CEMP abundances. At the lowest metallicities ([Fe/H] ≲ -4.0) we find the yields to contain ∼1 to 2 dex too much carbon, in agreement with all previous studies. At higher metallicities ([Fe/H] ∼ -3.0), where the observed data set is much larger, all our models produce yields with [C/Fe] values consistent with those observed in the most C-rich CEMPs. However it is only the low-mass models that undergo the Dual Shell Flash (which occurs at the start of the TPAGB) that can best reproduce the C and N observations. Normal Third Dredge-Up can not reproduce the observations because at these metallicities intermediate mass models (M ≳ 2 M⊙) suffer HBB which converts the C to N thus lowering [C/N] well below the observations, whilst if TDU were to occur in the low-mass (M ≤ 1 M⊙) models (we do not find it to occur in our models), the yields would be expected to be C-rich only, which is at odds with the "dual pollution" of C and N generally observed in the CEMPs. Interestingly events similar to the EMP Dual Flashes have been proposed to explain objects similarly containing a dual pollution of C and N - the "Blue Hook" stars and the "Born Again AGB" stars. We also find that the proportion of CEMP stars should continue to increase at lower metallicities, based on the results that some of the low mass EMP models already have polluted surfaces by the HB phase, and that there are more C-producing evolutionary episodes at these metallicities. Finally we note that there is a need for multidimensional fluid dynamics calculations of the Dual Flash events, to ascertain whether the overproduction of C and N at ultra-low metallicities found by all studies is an artifact of the 1D treatment. © ESO 2008.