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Register a new userEvolution and Nucleosynthesis of Extremely Metal Poor & Metal-Free Low- and Intermediate-Mass Stars I: Stellar Yield Tables and the CEMPs
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Simon Wattana Campbell
Publication date
2009
fields
Physics
and research's language is
English
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[Abridged] 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 Msun, amounting to 20 stars in total. In this paper, the first of a series describing and analysing this large data set, we present the resulting stellar yields. Many of the models experience violent nuclear burning episodes not seen at higher metallicities. We refer to these events as `Dual Flashes. These events have also 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 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 Msun) 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 Msun) 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.
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The origin of carbon-enhanced metal-poor (CEMP) stars plays a key role in characterising the formation and evolution of the first stars and the Galaxy since the extremely-poor (EMP) stars with [Fe/H] leq -2.5 share the common features of carbon enhancement in their surface chemical compositions. The origin of these stars is not yet established due to the controversy of the origin of CEMP stars without the enhancement of s-process element abundances, i.e., so called CEMP-no stars. In this paper, we elaborate the s-process nucleosynthesis in the EMP AGB stars and explore the origin of CEMP stars. We find that the efficiency of the s-process is controlled by O rather than Fe at [Fe/H] lesssim -2. We demonstrate that the relative abundances of Sr, Ba, Pb to C are explained in terms of the wind accretion from AGB stars in binary systems.
The evolution and explosion of metal-free stars with masses 10--100 solar masses are followed, and their nucleosynthetic yields, light curves, and remnant masses determined. When the supernova yields are integrated over a Salpeter initial mass function, 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. 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. 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. (abbreviated)
We study the evolution of extremely metal-poor AGB stars, with metallicities down to [Fe/H]=-5, to understand the main evolutionary properties, the efficiency of the processes able to alter their surface chemical composition and to determine the gas and dust yields. We calculate two sets of evolutionary sequences of stars in the 1-7.5Msun mass range, evolved from the pre-main sequence to the end of the AGB phase. To explore the extremely metal-poor chemistries we adopted the metallicities Z=3x10^{-5} and Z=3x10^{-7} which correspond, respectively to [Fe/H]=-3 and [Fe/H]=-5. The results from stellar evolution modelling are used to calculate the yields of the individual chemical species. We also modelled dust formation in the wind, to determine the dust produced by these objects. The evolution of AGB stars in the extremely metal-poor domain explored here proves tremendously sensitive to the initial mass of the star. M<2Msun stars experience several third dredge-up events, which favour the gradual surface enrichment of C12 and the formation of significant quantities of carbonaceous dust, of the order of 0.01Msun. The C13 and nitrogen yiel are found to be significantly smaller than in previous explorations of low-mass, metal-poor AGB stars, owing to the weaker proton ingestion episodes experienced during the initial AGB phases. M>5Msun stars experience hot bottom burning and their surface chemistry reflects the equilibria of a very advanced proton-capture nucleosynthesis; little dust production takes place in their wind. Intermediate mass stars experience both third dredge-up and hot bottom burning: they prove efficient producers of nitrogen, which is formed by proton captures on C12 nuclei of primary origin dredged-up from the internal regions.
We present the results of binary population simulations of carbon-enhanced metal-poor (CEMP) stars. We show that nitrogen and fluorine are useful tracers of the origin of CEMP stars, and conclude that the observed paucity of very nitrogen-rich stars puts strong constraints on possible modifications of the initial mass function at low metallicity. The large number fraction of CEMP stars may instead require much more efficient dredge-up from low-metallicity asymptotic giant branch stars.
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