Heavy elements in Globular Clusters: the role of AGB stars (original) (raw)
Related papers
Evolution and Nucleosynthesis of Extremely Metal-Poor AGB Stars
Context. Models of primordial and hyper-metal-poor stars that have masses similar to the Sun are known to experience an ingestion of protons into the hot core during the core helium flash phase at the end of their red giant branch evolution. This produces a concurrent secondary flash powered by hydrogen burning that gives rise to further nucleosynthesis in the core. Aims. We aim to model the nucleosynthesis occurring during the proton ingestion event to ascertain if any significant neutron-capture nucleosynthesis occurs. Methods. We perform post-process nucleosynthesis calculations on a one-dimensional stellar evolution calculation of a star with mass 1 M ⊙ and a metallicity of [Fe/H] = −6.5 that suffers a proton ingestion episode. Our network includes 320 nuclear species and 2,366 reactions and treats mixing and burning simultaneously. Results. We find that the mixing and burning of protons into the hot convective core leads to the production of 13 C, which then burns via the 13 C(α,n) 16 O reaction releasing a large number of free neutrons. During the first two years of neutron production the neutron poison 14 N abundance is low, allowing the prodigious production of heavy elements such as strontium, barium, and lead via slow neutron captures (the s process). These nucleosynthetic products are later mixed to the stellar surface and ejected via stellar winds. We compare our results with observations of the hyper-metal-poor halo star HE 1327-2326, which shows a strong Sr overabundance. Conclusions. Our model provides the possibility of self-consistently explaining the Sr overabundance in HE 1327-2326 together with its C, N, and O overabundances (all within a factor of ∼ 4) if the material were heavily diluted, for example, via mass transfer in a wide binary system. The model produces at least 18 times too much Ba than observed, but this may be within the large modelling uncertainties. In this scenario, binary systems of low mass must have formed in the early Universe. If true then this puts constraints on the primordial initial mass function.
Monthly Notices of the Royal Astronomical Society, 2014
We present a new grid of stellar models and nucleosynthetic yields for super-AGB stars with metallicities Z=0.001 and 0.0001, applicable for use within galactic chemical evolution models. Contrary to more metal rich stars where hot bottom burning is the main driver of the surface composition, in these lower metallicity models the effect of third dredge-up and corrosive second dredge-up also have a strong impact on the yields. These metal-poor and very metal-poor super-AGB stars create large amounts of 4 He, 13 C and 14 N, as well as the heavy magnesium isotopes 25 Mg and 26 Mg. There is a transition in yield trends at metallicity Z≈0.001, below which we find positive yields of 12 C, 16 O, 15 N, 27 Al and 28 Si, which is not the case for higher metallicities. We explore the large uncertainties derived from wind prescriptions in super-AGB stars, finding ≈ 2 orders of magnitude difference in yields of 22 Ne, 23 Na, 24,25,26 Mg, 27 Al and our s-process proxy isotope g. We find inclusion of variable composition low temperature molecular opacities is only critical for super-AGB stars of metallicities below Z≈0.001. We analyze our results, and those in the literature, to address the question: Are super-AGB stars the polluters responsible for extreme population in the globular cluster NGC 2808? Our results, as well as those from previous studies, seem unable to satisfactorily match the extreme population in this globular cluster.
Neutron capture nucleosynthesis in AGB stars
Astrophysical implications of the laboratory study of presolar materials, 1997
Recent AGB models including diffusive overshoot or rotational effects suggest the partial mixing (PM) of protons from the H-rich envelope into the C-rich layers during the third dredge-up. In order to study the impact of such a mixing on the surface abundances, nucleosynthesis calculations based on stellar AGB models are performed for different assumptions of protons (ranging from X mix p = 10 −6 to 0.7) in the PM zone. For high proton-to-12 C abundance ratios, light nuclei such as fluorine and sodium are efficiently produced, while heavier sprocess nuclei are synthesized for lower proton-to-12 C ratios. In the framework of the PM model, assuming a smooth exponentially decreasing proton profile, the surface 19 F abundance evolution is correlated with that of s-process nuclei in agreement with observations. However, as a function of the surface C/O abundance ratio, the surface 19 F enrichment remains difficult to reconcile with observations in AGB stars. Sodium is predicted to be efficiently produced in a small region of the PM zone with proton-to-12 C abundance ratio of about 10, but with large overproduction factors (up to fifty times higher than the sodium left over by the hydrogen burning shell). The primary 13 C pocket formed in the PM zone at low proton-to-12 C ratios is responsible for an efficient production of s-process nuclei. A table of elemental overabundances predicted at the surface of AGB stars at four different metallicities is presented. All the nucleosynthesis calculations are shown to suffer from major nuclear reaction rate uncertainties, in particular, 13 C (p , γ) 14 N , 14 N (n , p) 14 C and 22 Ne (α , n) 25 Mg. The major uncertainties associated with the amount of protons mixed into the C-rich zone are found in the extent of the PM zone rather than in the adopted H profile. Finally, the PM scenario predicts that low-metallicity AGB stars enriched in s-process elements should exhibit a large overproduction of Pb and Bi compared to other s-isotopes. The search of such Pb-stars is highly encouraged.
2001
We present the results of s-process nucleosynthesis calculations for AGB stars of different metallicities and initial masses. The computations were based on previously published stellar evolutionary models that account for the III dredge up phenomenon occurring late on the AGB. Neutron production is driven by the 13C(alpha,n)16O reaction during the interpulse periods in a tiny layer in radiative equilibrium at the top of the He- and C-rich shell. The s-enriched material is subsequently mixed with the envelope by the III dredge up, and the envelope composition is computed after each thermal pulse. We follow the changes in the photospheric abundance of the Ba-peak elements (heavy s, or `hs') and that of the Zr-peak ones (light s, or `ls'), whose logarithmic ratio [hs/ls] has often been adopted as an indicator of the s-process efficiency. The theoretical predictions are compared with published abundances of s elements for Galactic AGB giants of classes MS, S, SC, post-AGB super...
AGB yields and Galactic Chemical Evolution: last updated
We study the s-process abundances at the epoch of the Solar-system formation as the outcome of nucleosynthesis occurring in AGB stars of various masses and metallicities. The calculations have been performed with the Galactic chemical evolution (GCE) model presented by . With respect to previous works, we used updated solar meteoritic abundances, a neutron capture cross section network that includes the most recent measurements, and we implemented the s-process yields with an extended range of AGB initial masses. The new set of AGB yields includes a new evaluation of the 22 Ne(α, n) 25 Mg rate, which takes into account the most recent experimental information.
The Astrophysical Journal, 2014
Slow neutron captures at A 85 are mainly guaranteed by the reaction 13 C(α,n) 16 O in AGB stars, requiring proton injections from the envelope. These were so far assumed to involve a small mass (10 −3 M ⊙), but models with rotation suggest that in such tiny layers excessive 14 N hampers s-processing. Furthermore, s-element abundances in Galaxies require 13 C-rich layers substantially extended in mass (4×10 −3 M ⊙). We therefore present new calculations aiming at clarifying those issues and at understanding if the solar composition helps to constrain the 13 C "pocket" extension. We show: i) that mixing "from bottom to top" (like in magnetic buoyancy or other forced mechanisms) can form a 13 C reservoir substantially larger than assumed so far, covering most of the He-rich layers; ii) that stellar models at a fixed metallicity, based on this idea reproduce the main s-component as accurately as before; iii) that they make nuclear contributions from unknown nucleosynthesis processes (LEP P) unnecessary, against common assumptions. These models also avoid problems of mixing at the envelope border and fulfil requirements from C-star luminosities. They yield a large production of nuclei below A = 100, so that 86, 87 Sr may be fully synthesized by AGB stars, while 88 Sr, 89 Y and 94 Zr are contributed more efficiently than before. We finally suggest tests suitable to say a final word on the extension of the 13 C pocket.
s-Process abundances in AGB stars at various metallicities and their theoretical interpretation
Nuclear Physics A, 1997
Results from existing models of s-processing in red giants are compared with key observed abundances in population I and II AGB stars. Population I giants are particularly important for getting constraints on the neutron density (from Rb/Sr ratios), while population II AGB's provide clues to understand how the neutron exposure is achieved (through the ratio between Ba-peak and Sr-peak elements). AGB stars are shown to require s-processing with a very low neutron density, and producing very high Ba/Sr ratios at low metallicities. Both features are typical of radiative 13C-burning phases in AGB stars.
Super and Massive AGB Stars: II- Nucleosynthesis and
2013
We have computed detailed evolution and nucleosynthesis models for super and massive 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 alter 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 4 He, 7 Li (depending on the mass-loss formulation) 13 C, 14 N, 17 O, 23 Na, 25 Mg, as well the radioactive isotope 26 Al in agreement with previous investigation. In addition our results show enrichment of 22 Ne, 26 Mg and 60 Fe, 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, 14 N, 7 Li and 27 Al.