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Register a new userNucleosynthesis in Early Stars
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Simon Wattana Campbell
Publication date
2003
fields
Physics
and research's language is
English
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We present preliminary results of stellar structure and nucleosynthesis calculations for some early stars. The study (still in progress) seeks to explore the expected chemical signatures of second generation low- and intermediate-mass stars that may have formed out of a combination of Big Bang and Pop III (Z=0) supernovae material. Although the study is incomplete at this stage, we find some important features in our models. The initial chemical composition of these early stars is found to be significantly different to that given by just scaling the solar composition. The most notable difference is the lack of nitrogen. This should not affect the structural evolution significantly as nitrogen will be quickly produced through the CNO cycle due to the presence of carbon (and oxygen). It should however effect the nucleosynthetic yields. We also find that our very low metallicity five solar-mass model, with [Fe/H]=-4.01, does not reach the RGB - it goes directly to the helium burning phase. It does not experience the first dredge-up either. This is not a new finding but it will have an effect on the surface chemical evolution of the stars and should alter the nucleosynthetic yields that we are currently calculating. Our higher metallicity stars, with a globular cluster composition at [Fe/H]= -1.40, do experience all the standard phases of evolution but also have significantly higher surface temperatures and luminosities compared to solar metallicity stars. Their internal temperatures are also higher which should again effect the final nucleosynthetic yields.
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The first massive stars triggered the onset of chemical evolution by releasing the first metals (elements heavier than helium) in the Universe. The nature of these stars and how the early chemical enrichment took place is still largely unknown. Rotational-induced mixing in the stellar interior can impact the nucleosynthesis during the stellar life of massive stars and lead to stellar ejecta having various chemical compositions. We present low and zero-metallicity 20, 25 and 40 $M_{odot}$ stellar models with various initial rotation rates and assumptions for the nuclear reactions rates. With increasing initial rotation, the yields of light (from $sim$ C to Al) and trans-iron elements are boosted. The trans-iron elements (especially elements heavier than Ba) are significantly affected by the nuclear reaction uncertainties. The chemical composition of the observed CEMP (carbon-enhanced metal-poor) stars CS29528-028 and HE0336+0113 are consistent with the chemical composition of the material ejected by a fast rotating 40~$M_{odot}$ model.
Recent observations of r-process-enriched metal-poor star abundances reveal a non-uniform abundance pattern for elements $Zleq47$. 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 $n_{n}leq10^{13}$ $cm^{-3}$ or in the range $n_{n}geq10^{24}$ $cm^{-3}$ best explain the observations.
The first metal enrichment in the universe was made by supernova (SN) explosions of population (Pop) III stars. The trace remains in abundance patterns of extremely metal-poor (EMP) stars. We investigate the properties of nucleosynthesis in Pop III SNe by means of comparing their yields with the abundance patterns of the EMP stars. We focus on (1) jet-induced SNe with various energy deposition rates [$dot{E}_{rm dep}=(0.3-1500)times10^{51}{rm ergs s^{-1}}$], and (2) SNe of stars with various main-sequence masses ($M_{rm ms}=13-50M_odot$) and explosion energies [$E=(1-40)times10^{51}$ergs]. The varieties of Pop III SNe can explain varieties of the EMP stars: (1) higher [C/Fe] for lower [Fe/H] and (2) trends of abundance ratios [X/Fe] against [Fe/H].
We examine nucleosynthesis in the electron capture supernovae of progenitor AGB stars with an O-Ne-Mg core (with the initial stellar mass of 8.8 M_odot). Thermodynamic trajectories for the first 810 ms after core bounce are taken from a recent state-of-the-art hydrodynamic simulation. The presented nucleosynthesis results are characterized by a number of distinct features that are not shared with those of other supernovae from the collapse of stars with iron core (with initial stellar masses of more than 10 M_odot). First is the small amount of 56Ni (= 0.002-0.004 M_odot) in the ejecta, which can be an explanation for observed properties of faint supernovae such as SNe 2008S and 1997D. In addition, the large Ni/Fe ratio is in reasonable agreement with the spectroscopic result of the Crab nebula (the relic of SN 1054). Second is the large production of 64Zn, 70Ge, light p-nuclei (74Se, 78Kr, 84Sr, and 92Mo), and in particular, 90Zr, which originates from the low Y_e (= 0.46-0.49, the number of electrons per nucleon) ejecta. We find, however, that only a 1-2% increase of the minimum Y_e moderates the overproduction of 90Zr. In contrast, the production of 64Zn is fairly robust against a small variation of Y_e. This provides the upper limit of the occurrence of this type of events to be about 30% of all core-collapse supernovae.
We present the first evolutionary models of intermediate mass stars up to their thermal pulses which include effects of rotation on the stellar structure as well as rotationally induced mixing of chemical species and angular momentum. We find a significant angular momentum transport from the core to the hydrogen-rich envelope and obtain a white dwarf rotation rate comparable to current observational upper limits of 50 km/s. Large angular momentum gradients at the bottom of the convective envelope and the tip of the pulse driven convective shell are shown to produce marked chemical mixing between the proton-rich and the 12C-rich layers during the so called third dredge-up. This leads to a subsequent production of 13C which is followed by neutron production through 13C(alpha,n) in radiative layers in between thermal pulses. Although uncertainties in the efficiency of rotational mixing processes persist, we conclude that rotation is capable of producing a 13C-rich layer as required for the occurrence of the s-process in TP-AGB stars.
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