[en] Superbursts are energetic events on neutron stars that are a thousand times more powerful than ordinary type I X-ray bursts. They are believed to be powered by a thermonuclear explosion of accumulated ¹²C. However, the source of this ¹²C remains elusive to theoretical calculations and its concentration and ignition depth are both unknown. Here we present the first computational simulations of the nucleosynthesis during the thermal decay of a superbust, where X-ray bursts are quenched. Our calculations of the quenching time verify previous analytical calculations and shed new light on the physics of stable burning at low accretion rates. We show that concentrated (X¹²_C ∼> 0.40), although insufficient, amounts of ¹²C are generated during the several weeks following the superburst where the decaying thermal flux of the superburst stabilizes the burning of the accreted material.

[en] We discuss implications of recent precision measurements for the ⁹³Rh proton separation energy for the production of the lightest molybdenum isotopes in proton-rich type II supernova ejecta. It has recently been shown that a novel neutrino-induced process makes these ejecta a promising site for the production of the light molybdenum isotopes and other 'p-nuclei' with atomic mass near 100. The origin of these isotopes has long been uncertain. A distinguishing feature of nucleosynthesis in neutrino-irradiated outflows is that the relative production of ⁹²Mo and ⁹⁴Mo is set by a competition governed by the proton separation energy of ⁹³Rh. We use detailed nuclear network calculations and the recent experimental results for this proton separation energy to place constraints on the outflow characteristics that produce the lightest molybdenum isotopes in their solar proportions. It is found that for the conditions calculated in recent two-dimensional supernova simulations, and also for a large range of outflow characteristics around these conditions, the solar ratio of ⁹²Mo to ⁹⁴Mo cannot be achieved. This suggests that either proton-rich winds from type II supernova do not exclusively produce both isotopes, or that these winds are qualitatively different than calculated in today's supernova models

[en] We describe the interplay between nucleosynthesis, convection, accretion, and the resulting composition on the surface of an accreting standard neutron star as a function of the accretion rate and find the dependence of the critical accretion rate on the core luminosity

[en] We have used large-scale shell-model diagonalization calculations to determine the level spectra, proton spectroscopic factors, and electromagnetic transition probabilities for proton rich nuclei in the mass range A=44-63. Based on these results and the available experimental data, we calculated the resonances for proton capture reactions on neutron deficient nuclei in this mass range. We also calculated the direct capture processes on these nuclei in the framework of a Woods-Saxon potential model. Taking into account both resonant and direct contributions, we determined the ground-state proton capture reaction rates for these nuclei under hot hydrogen burning conditions for temperatures between 10⁸ and 10¹⁰ K. The calculated compound-nucleus level properties and the reaction rates are presented here; the rates are also available in computer-readable format from the authors

[en] We evaluate two dominant nuclear reaction rates and their uncertainties that affect ⁴⁴Ti production in explosive nucleosynthesis. Experimentally we develop thick-target yields for the ⁴⁰Ca(α,γ)⁴⁴Ti reaction at E_α = 4.13, 4.54, and 5.36 MeV using γ-ray spectroscopy. At the highest beam energy, we also performed an activation measurement which agrees with the thick target result. From the measured yields a stellar reaction rate was developed that is smaller than current statistical-model calculations and recent experimental results, which would suggest lower ⁴⁴Ti production in scenarios for the α-rich freeze out. Special attention has been paid to assessing realistic uncertainties of stellar reaction rates produced from a combination of experimental and theoretical cross sections. With such methods, we also develop a re-evaluation of the ⁴⁴Ti(α,p)⁴⁷V reaction rate. Using these two rates we carry out a sensitivity survey of ⁴⁴Ti synthesis in eight expansions representing peak temperature and density conditions drawn from a suite of recent supernova explosion models. Our results suggest that the current uncertainty in these two reaction rates could lead to as large an uncertainty in ⁴⁴Ti synthesis as that produced by different treatments of stellar physics.

[en] Nuclear physics is a basic ingredient in a large number of energetic astrophysical environments which are characterized by sufficient temperatures and densities to permit scattering events among particles, leading to nuclear reactions and possibly the production of unstable reaction products. Strong, electromagnetic and weak interactions (fusion, exchange reactions, photodisintegrations, beta-decays, electron [and positron] captures on nucleons and nuclei, neutrino scattering and captures [i.e. neutral and charged current reactions]) can produce nuclei far form stability and require extended knowledge of nuclear structure near and far from stability, including decay and fission properties. Last, but not least, the nucleon-nucleon interaction determines the nuclear equation of state at and beyond nuclear densities and is ultimately connected to the question under which conditions a phase transitions from hadrons to the quark-gluon plasma occurs. In this review we will survey how these aspects of nuclear physics enter the modeling of astrophysical objects

[en] The rp-process has been suggested as the dominant nucleosynthesis process in explosive hydrogen burning at high temperature and density conditions. The process is characterized by a sequence of fast proton capture reactions and subsequent β-decays. The reaction path of the rp-process runs along the drip line up to Z∼50. Most of the charged-particle reaction rates for the reaction path are presently based on statistical Hauser-Feshbach calculations. While these rates are supposed to be reliable within a factor of two for conditions of high density in the compound nuclei, discrepancies may occur for nuclei near closed shells or near the proton drip line where the Q-values of proton capture processes are typically very small. It has been argued that the thermonuclear runaway is less sensitive to the reaction rates because of the rapid time-scale of the event. However, since these processes may operate at the same time-scale as fast mixing and convection processes, a change in reaction rates indeed may have a significant impact. In this paper we present two examples, the break-out from the hot CNO cycles, and the thermonuclear runaway in X-ray bursts itself, where changes in reaction rates have a direct impact on time-scale, energy generation and nucleosynthesis predictions for the explosive event. (orig.)

[en] This paper reports on the detailed rp-process reaction flow on an accreting neutron star and the resulting ashes of a type I X-ray burst. It is obtained by coupling a 298 isotope reaction network to a self-consistent one-dimensional model calculation with a constant accretion rate of M-bar =1017 g/s

[en] The light curves of type I X-ray bursts (XRBs) result from energy released from the atmosphere of a neutron star when accreted hydrogen and helium ignite and burn explosively via the rp-process. Since charged particle reaction rates are both density and very temperature dependent, a simulation model must provide accurate values of these variables to predict the reaction flow. This paper uses a self-consistent one-dimensional model calculation with a constant accretion rate of M radical =5x10¹⁶g/s(0.045M radical_Ed.) and reports on the detailed rp-process reaction flow of a given burst