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We investigate the quantitative constraint on the triple-α reaction rate based on stellar evolution theory, motivated by the recent significant revision of the rate proposed by nuclear physics calculations. Targeted stellar models were computed in order to investigate the impact of that rate in the mass range of 0.8≤M/M⊙≤25 and in the metallicity range between Z = 0 and Z = 0.02. The revised rate has a significant impact on the evolution of low-and intermediate-mass stars, while its influence on the evolution of massive stars (M > 10M⊙) is minimal. We find that employing the revised rate suppresses helium shell flashes on AGB phase for stars in the initial mass range 0.8≤M/M⊙≤6, which is contradictory to what is observed. The absence of helium shell flashes is due to the weak temperature dependence of the revised triple-α reaction cross section at the temperature involved. In our models, it is suggested that the temperature dependence of the cross section should have at least ν > 10 at T = 1-1.2×108K where the cross section is proportional to Tν. We also derive the helium ignition curve to estimate the maximum cross section to retain the low-mass first red giants. The semi-analytically derived ignition curves suggest that the reaction rate should be less than ∼ 10-29 cm6 s-1 mole-2 at ≈ 107.8 K, which corresponds to about three orders of magnitude larger than that of the NACRE compilation. [ABSTRACT FROM AUTHOR]

We investigate the effect of the triple-α reaction rates on the evolution of low-mass stars and massive stars. The former is compared with the observations of metal-poor stars known to date. For the latter, we discuss the impact of recent calculation of triple-α reaction rate by Ogata et al. (2009, PTP, 122, 1055) on the evolution until carbon burning. [ABSTRACT FROM AUTHOR]

The information on surface abundances in extremely metal-poor (EMP) stars in the Galactic halo is a useful probe of the early universe. Therefore, it is very important to understand the modification of abundances in EMP stars by internal and/or external pollution through mixing processes and binary interactions in stellar evolution. We computed various sets of model stars for 0.8–9.0–M⊙ in mass and -5< [Fe/H] <-2.3 and Z = 0 in metallicity. Evolution is followed from zero-age main sequence to the thermally pulsating AGB phase including the onset of hydrogen entrainment by the helium flash convection during the red giant branch phase and/or the asymptotic giant branch phase. We explored the evolutionary characteristics depending on the initial mass and metallicity. There exist various types of mixing episode of how the hydrogen mixing sets in and of how it affects the final abundances in the surface. For low mass models having M<1.2 M⊙, strong helium flash leads to the large enhancement of CN elements through the dredge-up of materials produced in the helium flash convective region that injected the protons in the hydrogen burning shell. For more massive models, several episodes of mixing occur, with weak mixing events at AGB phase, which may allow longer timescale for nucleosynthesis in the helium flash convective region. We also found a difference in the efficiencies of dredge-up during the thermally pulsating AGB phase. [ABSTRACT FROM AUTHOR]

In extremely metal-poor stars ([Fe H]< 2 5), hydrogen is mixed into the convection driven by helium flash and induces neutron-capture nucleosynthesis with the reactions, 12C(p γ)13N(e+ν)13C(α n)16O, as the neutron source. We investigate the progress of this nucleosynthesis with use of nuclear network for a wide range of model parameters such as the amount of mixed hydrogen and the strength of helium flash. We reveal the characteristic abundance pattern from the light elements through the s-process elements, produced by alpha- and neutron-capture reactions under the extremely metal-poor condition. On the basis, we explore the possible modifications of surface abundances in the metal-free, Population III stars and discuss their relevance to the two most iron-deficient stars, HE0107-5240 and HE1327-2326, known to date. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]

We construct a binary population synthesis model to explore the star formation history of the Galaxy. The model includes the effects of AGB evolution and binary mass transfer for a given IMF and binary period distribution function. We discuss the origins of extremely metal-poor stars with enhancement of carbon or nitrogen, with the possible effect of mass loss at low-metallicity taken into account. Our results strongly support high-mass dominated star formation during the early epoch of the Galaxy in order to explain the observed frequency of carbon and nitrogen enhancements that are thought to result from mass transfer from a former AGB binary companion. Our model also suggests that the IMF had a transition phase at [Fe/H] ∼ -2. [ABSTRACT FROM AUTHOR]

The evolution of extremely metal-poor (EMP) stars of low-/intermediate-masses is distinct from those of metal-rich stars in that the convection driven by the helium shell flash can extend outward into the hydrogen-rich layer during TP-AGB phase. In the circumstance of [Fe/H]<-2.5, protons are mixed and converted into neutrons in the convective zone to promote nucleosynthesis through neutron and α-captures. We study the nucleosynthesis in the helium-flash convective zone, induced by this hydrogen mixing. In the dearth of the pristine metals, the neutron-recycling reactions, 12C(n,γ)13C(α,n)16O, and in some cases, the subsequent 16O(n,γ)17O(α,n)20Ne, play an important role and catalyze the syntheses of O through Mg and still heavier elements. In particular, it is demonstrated that such peculiar abundance patterns of light elements from C through Al and heavy elements of Sr as observed from the two most iron-deficient stars, HE0107-5240 and HE1327-2326, can well be reproduced in terms of the nucleosynthesis in the metal-free and EMP AGB stars. In addition, the lack of Na and Al enhancement for a carbon-rich giant HE0557-4840 can be interpreted as the absence of neutron-capture reactions because of the negligible amount of proton ingestion. Based on these results, we assign their origin to the Pop III stars, born out of the primordial gas. We also discuss about the surface pollution both via the mass transfer in the binary systems and via the accretion of interstellar gas. [ABSTRACT FROM AUTHOR]

We investigate the nucleosynthesis in the helium flash convective zone, triggered by the hydrogen mixing, for extremely metal-poor stars of low and intermediate mass. Mixed hydrogen is converted into neutron through 12C(p,γ)13N(e+ν)13C(α,n)16O and the doubly neutron-recycling reactions 12C(n,γ)13C(α,n)16O(n,γ)17O(α,n)20Ne operate. In addition to oxygen and neon, not only light elements from sodium through phosphorus but also the s-process elements, heavier than iron, are synthesized via successive neutron captures with 20Ne as seeds even in the stars originally devoid of metals. We follow the both the doubly neutron-recycling reactions and the s-process nucleosynthesis up to Pb and Bi by varying model parameters such as the amount of mixed 13C. The resultant abundance patterns is shown to reproduce the observed enhancement not only of oxygen, the light elements but also Sr observed from HE 0107-5240 and HE 1327-2326. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]