Nucleosynthesis in extremely metal-poor intermediate-mass stars (original) (raw)
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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.
Proton-capture Nucleosynthesis In Low Mass Stars: Effects of New Reaction Rates
2011
We present computations of nucleosynthesis in low-mass (LM) red giant branch (RGB) and asymptotic giant branch (AGB) stars of Population I experiencing extended mixing. We adopt the updated version of the FRANEC evolutionary model, a new post-process code for non-convective mixing and the most recent revisions for solar abundances. In this framework, we discuss the effects of recent improvements in relevant reaction rates for proton captures on intermediate-mass (IM) nuclei (from carbon to aluminum). For each nucleus, we briefly discuss the new choices and their motivations. The calculations are then performed on the basis of a parameterized circulation, where the effects of the new nuclear inputs are best compared to previous works. We find that the new rates (and notably the one for the 14 N(p, γ ) 15 O reaction) imply considerable modifications in the composition of post-mainsequence stars. In particular, the slight temperature changes due to the reduced efficiency of proton captures on 14 N induce abundance variations at the first dredge-up (especially for 17 O, whose equilibrium ratio to 16 O is very sensitive to the temperature). In this new scenario, presolar oxide grains of AGB origin turn out to be produced almost exclusively by very low mass stars (M 1.5-1.7 M ), never becoming C-rich. The whole population of grains with 18 O/ 16 O below 0.0015 (the limit permitted by first dredge-up) is now explained. Also, there is now no forbidden area for very low values of 17 O/ 16 O (below 0.0005), contrary to previous findings. A rather shallow type of transport seems to be sufficient for the CNO changes in RGB stages. Both thermohaline diffusion and magnetic-buoyancy-induced mixing might provide a suitable physical mechanism for this. Thermohaline mixing is in any case certainly inadequate to account for the production of 26 Al on the AGB. Other transport mechanisms must therefore be at play. In general, observational constraints from RGB and AGB stars, as well as from presolar grains, are well reproduced by our approach. The nitrogen isotopic ratio in mainstream SiC grains remains an exception. For the low values measured in them (i.e., for 14 N/ 15 N 2000), we have no explanation. Actually, for the several grains with subsolar nitrogen isotopic ratios, no known stellar process acting in LM stars can provide a clue. This might be an evidence that some form of contamination from cosmic ray spallation occurs in the interstellar medium, adding fresh 15 N to the grains.
Neutron Capture Elements ins‐Process–Rich, Very Metal‐Poor Stars
The Astrophysical Journal, 2001
We report abundance estimates for neutron-capture elements, including lead (Pb), and nucleosynthesis models for their origin, in two carbon-rich, very metal-poor stars, LP 625-44 and LP 706-7. These stars are subgiants whose surface abundances are likely to have been strongly affected by mass transfer from companion AGB stars that have since evolved to white dwarfs. The detections of Pb, which forms the final abundance peak of the s-process, enable a comparison of the abundance patterns from Sr (Z = 38) to Pb (Z = 82) with predictions of AGB models. The derived chemical compositions provide strong constraints on the AGB stellar models, as well as on s-process nucleosynthesis at low metallicity. The present paper reports details of the abundance analysis for 16 neutron-capture elements in LP 625-44, including the effects of hyperfine splitting and isotope shifts of spectral lines for some elements. A Pb abundance is also derived for LP 706-7 by a re-analysis of a previously observed spectrum. We investigate the characteristics of the nucleosynthesis pathway that produces the abundance ratios of these objects using a parametric model of the s-process without adopting any specific stellar model. The neutron exposure τ is estimated to be about 0.7mb −1 , significantly larger than that which best fits solar-system material, but consistent with the values predicted by models of moderately metal-poor AGB stars. This value is strictly limited by the Pb abundance, in addition to those of Sr and Ba. We also find that the observed
Nucleosynthesis and Evolution of Massive Metal-Free Stars
The Astrophysical Journal, 2010
The evolution and explosion of metal-free stars with masses 10 − 100 M ⊙ are followed, and their nucleosynthetic yields, light curves, and remnant masses determined. Such stars would have been the first to form after the Big Bang and may have left a distinctive imprint on the composition of the early universe. When the supernova yields are integrated over a Salpeter initial mass function (IMF), the resulting elemental abundance pattern is qualitatively solar, but with marked deficiencies of odd-Z elements with 7 ≤ Z ≤ 13. Neglecting the contribution of the neutrino wind from the neutron stars that they make, no appreciable abundances are made for elements heavier than germanium. The computed pattern compares favorably with what has been observed in metal-deficient stars with [Z] −3. The amount of ionizing radiation from this generation of stars is ∼ 2.16 MeV per baryon (4.15 B per M ⊙ ; where 1 B = 1 Bethe = 10 51 erg) for a Salpeter IMF, and may have played a role in reionizing the universe. Most of the stars end their lives as blue supergiants and make supernovae with distinctive light curves resembling SN 1987A, but some produce primary nitrogen by dredge up and become red supergiants. These make brighter supernovae like typical Type IIp's. For the lower mass supernovae considered, the distribution of remnant masses clusters around typical modern neutron star masses, but above 20 M ⊙ to 30 M ⊙ , with the value depending on explosion energy, black holes are copiously formed by fallback, with a maximum hole mass of ∼ 40 M ⊙. A novel automated fitting algorithm is developed for determining optimal combinations of explosion energy, mixing, and initial mass function in the large model data base to agree with specified data sets. The model is applied to the low metallicity sample of Cayrel et al. (2004) and the two ultra-iron-poor stars HE0107-5240 and HE1327-2326. Best agreement with these low metallicity stars is achieved with very little mixing, and none of the metal-deficient data sets considered show the need for a high energy explosion component. To the contrary, explosion energies somewhat less than 1.2 B seem to be preferred in most cases.
We prospose a unified model for the nucleosynthesis of heavy (A > 57) elements in stars. The neutron flux can be set to describe neutron capture in arbitrary neutron flux. Our approach solves the coupled differential equations, that describe the neutron capture and decays of 2696 nuclei, numerically without truncating those to include only either capture or decay as traditionally assumed in weak neutron flux (s process). As a result the synthesis of heavy nuclei always evolves along a wide band in the valley of stable nuclei. The observed abundances in the Solar system are reproduced reasonably already in the simplest version of the model. The model predicts that the nucleosynthesis in weak or modest neutron flux produces elements that are traditionally assumed to result in the high neutron flux of supernovae explosions (r process).
S- and R-Process Elements in Two Very Metal-Poor Stars
2001
New measurements of neutron-capture elements are presented for two very metal-poor stars ([Fe/H] ∼ −3). One (LP 625-44) has an s-process signature believed to be due to mass transfer from a now-extinct metal-poor AGB companion, and the second (CS 22897-008) is one of a number of very metal-poor stars having high [Sr/Ba] ratios which is not expected from the r-process. In the sprocess star, many elements including lead have been detected, providing strong constraints on the 13 C pocket in the now-extinct AGB star. In the Sr-rich star, Zn, Y, and Zr are also seen to be overabundant, and several possible nucleosynthesis mechanisms are discussed.
2001
Except for 1 H, 2 H, 3 He, 4 He, and 7 Li, originating from the Big Bang, all heavier elements are made in stellar evolution and stellar explosions. Nuclear physics, and in many cases nuclear structure far from stability, enters in a crucial way. Therefore, we examine in this review the role of nuclear physics in astrophysics in general and in particular how it affects stellar events and the resulting nucleosynthesis. Stellar modeling addresses four major aspects: 1. energy generation and nucleosynthesis, 2. energy transport via conduction, radiation or possibly convection, 3. hydrodynamics/hydrostatics, and finally 4. thermodynamic properties of the matter involved. Nuclear Physics enters via nuclear reaction cross sections and nuclear structure (affecting the composition changes and nuclear energy generation), neutrinonucleon and neutrino-nucleus cross sections (affecting neutrino opacities and transport), and e.g. the equation of state at and beyond nuclear densities which creates a relation between the nuclear many body problem and and hydrodynamic response like pressure and entropy. In the following we review these four topics by highlighting the role and impact of nuclear physics in each of these aspects of stellar modeling. The main emphasis is put on the connection to element synthesis.
Nucleosynthesis in the early Galaxy
2008
Recent observations of r-process-enriched metal-poor star abundances reveal a non-uniform abundance pattern for elements Z ≤ 47. Based on non-correlation trends between elemental abundances as a function of Eu-richness in a large sample of metal-poor stars, it is shown that the mixing of a consistent and robust light element primary process (LEPP) and the r-process pattern found in r-II metal-poor stars explains such apparent non-uniformity. Furthermore, we derive the abundance pattern of the LEPP from observation and show that it is consistent with a missing component in the solar abundances when using a recent s-process model. As the astrophysical site of the LEPP is not known, we explore the possibility of a neutron capture process within a site-independent approach. It is suggested that scenarios with neutron densities n n ≤ 10 13 cm −3 or in the range n n ≥ 10 24 cm −3 best explain the observations.