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Register a new userNucleosynthesis in Electron Capture Supernovae of AGB Stars
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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.
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Stars of $sim$ 8 - 10 $M_{odot}$ on the main-sequence form strongly electron-degenerate O+Ne+Mg core and become super-AGB stars. If such an O+Ne+Mg core grows to 1.38 $M_odot$, electron captures on $^{20}$Ne$(e, u_e)^{20}$F$(e, u_e)^{20}$O take place and ignite O-Ne deflagration around the center. In this paper, we perform two-dimensional hydrodynamics simulations of the propagation of the O-Ne flame to see whether such a flame induces a collapse of the O+Ne+Mg core due to subsequent electron capture behind the flame or triggers a thermonuclear explosion. We present a series of models to explore how the outcome depends on model parameters for the central density in the range from $10^{9.80}$ to $10^{10.20}$ g cm$^{-3}$, flame structure of both centered and off-centered ignition kernels, special and general relativistic effects, turbulent flame speed formula and the treatments of laminar burning phase. We find that the O+Ne+Mg core obtained from stellar evolutionary models has a high tendency to collapse into a neutron star. We obtain the bifurcation between the electron-capture collapse and thermonuclear explosion. We discuss the implication in nucleosynthesis and the possible observational signals of this class of 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.
Based on evolutionary computations of 90 stellar models, we have analysed the impact of initial composition and core overshooting on the post-He-burning evolution and the associated nucleosynthesis of Super-AGB stars, pointing particular attention on the C-burning phase. Moreover the possible link between the transition masses $M_{up}$, $M_{N}$ and $M_{mas}$ (defined as the critical initial mass above which C-burning ignites, the minimum initial mass for an electron-capture supernova and the minimum initial mass for the completion of all the nuclear burning phases respectively) and the properties of the core during the core He-burning phase is also briefly discussed.
In this paper we present a large-scale sensitivity study of reaction rates in the main component of the $s$ process. The aim of this study is to identify all rates, which have a global effect on the $s$ process abundance distribution and the three most important rates for the production of each isotope. We have performed a sensitivity study on the radiative $^{13}$C-pocket and on the convective thermal pulse, sites of the $s$ process in AGB stars. We identified 22 rates, which have the highest impact on the $s$-process abundances in AGB stars.
Electron capture rates on neutron-rich nuclei (A>65) were calculated within the Random Phase Approximation with partial number formalism, including allowed and forbidden transitions. The partial occupation numbers were provided as a function of temperature by Shell-Model Monte Carlo calculations, including an pairing+quadrupole interaction. Capture rates on relevent nuclei were calculated for density and temperature conditions during the core collapse of a massive star. It was found that electron captures on nuclei can compete with electron captures on free protons. Furthermore, they produce neutrinos with average energies lower than neutrinos emitted from captures on free protons, with possible consequences on the cooling of the core.
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