Complete nucleosynthesis calculations for low-mass stars from NuGrid
Marco Pignatari, Falk Herwig, Michael Bennett, Steven Diehl, Christopher L. Fryer, Raphael Hirschi, Aimee Hungerford, Georgios Magkotsios, Gabriel Rockefeller, … 展开作者
Marco Pignatari, Falk Herwig, Michael Bennett, Steven Diehl, Christopher L. Fryer, Raphael Hirschi, Aimee Hungerford, Georgios Magkotsios, Gabriel Rockefeller, Francis X. Timmes, Patrick Young
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To appear in the Conference Proceedings for the "10th Symposium on Nuclei in the Cosmos (NIC X)", July 27 - August 1 2008, Mackinack Island, Michigan, USA
Many nucleosynthesis and mixing processes of low-mass stars as they evolve from the Main Sequence to the thermal-pulse Asymptotic Giant Branch phase (TP-AGB) are well understood (although of course important physics components, e.g. rotation, magnetic fields, gravity wave mixing, remain poorly known). Nevertheless, in the last years presolar grain measurements with high resolution have presented new puzzling problems and strong constraints on nucleosynthesis processes in stars. The goal of the NuGrid collaboration is to present uniform yields for a large range of masses and metallicities, including low$-$mass stars and massive stars and their explosions. Here we present the first calculations of stellar evolution and high-resolution, post-processing simulations of an AGB star with an initial mass of 2 M_sun and solar-like metallicity (Z=0.01), based on the post-processing code PPN. In particular, we analyze the formation and evolution of the radiative 13C-pocket between the 17th TP and the 18th TP. The s-process nucleosynthesis profile of a sample of heavy isotopes is also discussed, before the next convective TP occurrence.
Simulations of nucleosynthesis in astrophysical environments are at the intersection of nuclear physics reaction rate research and astrophysical applications, for example in the area of galactic chemical evolution or near-field cosmology. Unfortunately, at present the available yields for such applications are based on heterogeneous assumptions between the various contributing nuclear production sites, both in terms of modeling the thermodynamic environment itself as well as the choice of specifc nuclear reaction rates and compilations. On the other side, new nuclear reaction rate determinatio…
The s-process production in massive stars at very low metallicities is expected to be negligible due to the low abundance of the neutron source 22Ne, to primary neutron poisons and decreasing iron seed abundances. However, recent models of massive stars including the effects of rotation show that a strong production of 22Ne is possible in the helium core, as a consequence of the primary nitrogen production (observed in halo metal poor stars). Using the PPN post-processing code, we studied the impact of this primary 22Ne on the s process. We find a large production of s elements between stronti…
We present preliminary results from recent high-resolution double-degenerate merger simulations with the Smooth Particle Hydrodynamics (SPH) technique. We put particular emphasis on verification and validation in our effort and show the importance of details in the initial condition setup for the final outcome of the simulation. We also stress the dynamical importance of including shocks in the simulations. These results represent a first step toward a suite of simulations that will shed light on the question whether double-degenerate mergers are a viable path toward type 1a supernovae. In fut…
We review some of the uncertainties in calculating nucleosynthetic yields, focusing on the explosion mechanism. Current yield calculations tend to either use a piston, energy injection, or enhancement of neutrino opacities to drive an explosion. We show that the energy injection, or more accurately, an entropy injection mechanism is best-suited to mimic our current understanding of the convection-enhanced supernova engine. The enhanced neutrino-opacity technique is in qualitative disagreement with simulations of core-collapse supernovae and will likely produce errors in the yields. But piston-…
The r-process constitutes one of the major challenges in nuclear astrophysics. Its astrophysical site has not yet been identified but there is observational evidence suggesting that at least two possible sites should contribute to the solar system abundance of r-process elements and that the r-process responsible for the production of elements heavier than Z=56 operates quite robustly producing always the same relative abundances. From the nuclear-physics point of view the r-process requires the knowledge of a large number of reaction rates involving exotic nuclei. These include neutron captur…
There have been attempts to fit the abundance patterns of extremely metal-poor stars with supernova nucleosynthesis models for the lighter elements than Zn. On the other hand, observations have revealed that the presence of EMP stars with peculiarly high ratio of "weak r-process elements" Sr, Y and Zr. Although several possible processes were suggested for the origin of these elements, the complete solution for reproducing those ratios is not found yet. In order to reproduce the abundance patterns of such stars, we investigate a model with neutron rich matter ejection from the inner region of …
The structural and nucleosynthetic evolution of 3, 4, 5, 6 and \mass{7} stars with two metallicities ($Z = 0.005$ and 0.02) has been computed in detail, from the early pre-main sequence phase up to the thermally pulsing (TP) AGB phase or the onset of off-center carbon burning. Typically 10 to 20 thermal pulses have been followed for each TP-AGB object. This homogeneous and quite large set of models allows us to present an overview of the thermal pulse properties as well as of the nucleosynthesis accompanying the TP-AGB phase of intermediate-mass stars. More specifically, after a brief descript…
We calculate presupernova evolutions and supernova explosions of massive stars (M=13-25 Mo) for various metallicities. We find the following characteristic abundance patterns of nucleosynthesis in the metal-free (Pop III) stars. (1) The alpha-nuclei (from C to Zn) are more efficiently produced than other isotopes, and the abundance pattern of alpha-nuclei can be similar to the solar abundance. In particular, near solar ratios of alpha elements/Fe might be a signature of Pop III which could produce a large amount of Fe. (2) The abundance ratios of odd Z to even Z elements such as Na/Mg and Al/M…
We calculate nucleosynthesis in core-collapse explosions of massive Pop III stars, and compare the results with abundances of metal-poor halo stars to constrain the parameters of Pop III supernovae. We focus on iron-peak elements and, in particular, we try to reproduce the large [Zn/Fe] observed in extremely metal-poor stars. The interesting trends of the observed ratios [Zn, Co, Mn, Cr, V/Fe] can be related to the variation of the relative mass of the complete and incomplete Si-burning regions in supernova ejecta. We find that [Zn/Fe] is larger for deeper mass-cuts, smaller neutron excess, an…
We discuss the synthesis of some short-lived isotopes and of 23Na in thermally pulsing AGB stars with initial mass of 2 Msun and two different metallicities (Z=1.5e-2, corresponding to the metal amount in the Sun, and Z=1e-4), representative of disk and halo stars, respectively. The different nucleosynthesis channels are illustrated in some details. As previously found, the 13C formed after each third dredge up episode is usually completely consumed by alpha captures before the onset of the subsequent thermal pulse, releasing neutrons. This is the most efficient neutron source in low mass AGB …
We present a comprehensive study of s-process nucleosynthesis in 15, 20, 25, and 30 $\msun$ stellar models having solar-like initial composition. The stars are evolved up to ignition of central neon with a 659 species network coupled to the stellar models. In this way, the initial composition from one burning phase to another is consistently determined, especially with respect to neutron capture reactions. The aim of our calculations is to gain a full account of the s-process yield from massive stars. In the present work, we focus primarily on the s-process during central helium burning and il…
We investigate the evolution, final fate, and nucleosynthetic yields of rotating and non-rotating very massive stars (VMS) of zero metallicity. First we address the issue of mass loss during hydrogen burning due to vibrational instabilities. We find that these objects are much more stable than what was found in previous studies of VMS of solar composition, and expect only negligible mass loss driven by the pulsations. As these stars thus reach the end of their evolution with massive helium cores, they encounter the pair-creation instability. We find that for helium core masses of ~64...133 sol…
I discuss simulations of the coalescence of black hole neutron star binary systems with black hole masses between 14 and 20 \msun. The calculations use a three-dimensional smoothed particle hydrodynamics code, a temperature-dependent, nuclear equation of state and a multi-flavor neutrino scheme. General relativistic effects are mimicked using the \Pacz-Wiita pseudo-potential and gravitational radiation reaction forces. Opposite to previous, purely Newtonian calculations, in none of the explored cases episodic mass transfer occurs. The neutron star is always completely disrupted after most of i…
We study neutrino process nucleosynthesis in massive stars using newly calculated cross sections, an expanded reaction network, and complete and self-consistent models of the progenitor star. We reevaluate the production of light isotopes from abundant progenitors as well as that of rare, heavy, proton-rich isotopes. In particular, new results are given for B11, F19, La138, and Ta180. The production of these isotopes places limits on neutrino spectrum and oscialltions.
We discuss the oscillations effects on neutrinos in the early Universe and update the cosmological constraints on the oscillation parameters. It is shown that sterile LOW solution to the solar neutrino problem is almost completely excluded from cosmological nucleosynthesis considerations. Two possibilities for the relaxation of this constraint are discussed: high primordial helium-4 yield and a relic lepton asymmetry present at nucleosynthesis epoch. The numerical analysis proved that Y_p=0.25 only relaxes the constraint on LOW solution, while L > 10^{-5} is capable to remove it.
We report on an investigation into stellar evolution and nucleosynthesis in the low and extremely low metallicity regime, including models of stars with a pure Big Bang composition (i.e. Z=0). The metallicity range of the extremely metal poor (EMP) models we have calculated is -6.5 < [Fe/H] < -3.0, whilst our models are in the mass range 0.85 < M < 3.0 Msun. Many of the EMP and Z=0 models experience violent evolutionary episodes not seen at higher metallicities. We refer to these events as `Dual Flashes' since they are characterised by peaks in the hydrogen and helium burning luminosities occu…
In order to study the processes creating intermediate and heavy nuclei in massive stars it is necessary to provide neutron capture cross sections and reaction rates close to stability and for moderately unstable neutron-rich nuclei. Furthermore, one has to know the efficiency of neutron-releasing reactions in the main evolutionary phases of a massive star. We present simulations of the nucleosynthesis in a 15 and 25 solar mass star, for the first time followed completely from main sequence hydrogen burning until the type II supernova explosion including all nuclides up to Bi. Theoretical react…
It is widely accepted that moduli in the mass range 10eV - $10^4$GeV which start to oscillate with an amplitude of the order of the Planck scale either jeopardize successful predictions of nucleosynthesis or overclose the Universe. It is shown that the moduli problem can be relaxed by making use of parametric resonance. A new non-perturbative decay channel for moduli oscillations is discussed. This channel becomes effective when the oscillating field results in a net negative mass term for the decay products. This scenario allows for the decay of the moduli much before nucleosynthesis and, the…
The recently improved information on the stellar (n,gamma) cross sections of neutron-magic nuclei at N = 82, and in particular of 142Nd, turned out to represent a sensitive test for models of s-process nucleosynthesis. While these data were found to be incompatible with the classical approach based on an exponential distribution of neutron exposures, they provide significantly better agreement between the solar abundance distribution of s nuclei and the predictions of models for low mass AGB stars. Particular attention is paid to a consistent description of s-process branchings in the region o…
Nucleosynthesis restrictions on mixing of active neutrinos with possible sterile ones are obtained with the account of experimentally determined mixing between all active neutrinos. The earlier derived bounds, valid in the absence of active-active mixing, are reanalyzed and significant difference is found in the resonance case. The results are obtained both analytically and numerically by solution of complete system of integro-differential kinetic equations. A good agreement between analytical and numerical approaches is demonstrated. A role of possibly large cosmological lepton asymmetry is d…
