Abstract
The first-peak s-process elements Rb, Sr, Y and Zr in the post-AGB star Sakurai's object (V4334 Sagittarii) have been proposed to be the result of i-process nucleosynthesis in a post-AGB very-late thermal pulse event. We estimate the nuclear physics uncertainties in the i-process model predictions to determine whether the remaining discrepancies with observations are significant and point to potential issues with the underlying astrophysical model. We find that the dominant source in the nuclear physics uncertainties are predictions of neutron capture rates on unstable neutron rich nuclei, which can have uncertainties of more than a factor 20 in the band of the i-process. We use a Monte Carlo variation of 52 neutron capture rates and a 1D multi-zone post-processing model for the i-process in Sakurai's object to determine the cumulative effect of these uncertainties on the final elemental abundance predictions. We find that the nuclear physics uncertainties are large and comparable to observational errors. Within these uncertainties the model predictions are consistent with observations. A correlation analysis of the results of our MC simulations reveals that the strongest impact on the predicted abundances of Rb, Sr, Y and Zr is made by the uncertainties in the (n, γ) reaction rates of 85Br, 86Br, 87Kr, 88Kr, 89Kr, 89Rb, 89Sr, and 92Sr. This conclusion is supported by a series of multi-zone simulations in which we increased and decreased to their maximum and minimum limits one or two reaction rates per run. We also show that simple and fast one-zone simulations should not be used instead of more realistic multi-zone stellar simulations for nuclear sensitivity and uncertainty studies of convective–reactive processes. Our findings apply more generally to any i-process site with similar neutron exposure, such as rapidly accreting white dwarfs with near-solar metallicities.
Highlights
Most of the solar system chemical elements heavier than iron were produced in the slow (s) and rapid (r) neutron capture processes in previous generations of stars and stellar explosions (Kappeler et al, 2011; Thielemann et al, 2011)
In this paper we focus on the nuclear physics uncertainties in predicting the Rb, Sr, Y and Zr abundances in a full multi-zone post-AGB star model of relevance for Sakurai’s object and rapidly accreting white dwarfs (RAWDs), and identify the key reaction uncertainties that need to be improved in the future
Constant Temperature matched to the Fermi Gas (CT+BSFG) (Dilg et al, 1973) Back-shifted Fermi Gas (BSFG) (Dilg et al, 1973; Gilbert and Cameron, 1965) Generalized Super fluid (GSM) (Ignatyuk et al, 1979; Ignatyuk et al, 1993) Hartree Fock using Skyrme force (HFS) (Goriely et al, 2001) Hartree-Fock-Bogoliubov + combinatorial (HFBS-C) (Goriely et al, 2008)
Summary
Most of the solar system chemical elements heavier than iron were produced in the slow (s) and rapid (r) neutron capture processes in previous generations of stars and stellar explosions (Kappeler et al, 2011; Thielemann et al, 2011). The higher neutron density in the i process is achieved at typical Heburning temperatures T ∼ 2 – 3 × 108 K due to efficient replenishment of 13C from the reaction 12C(p,γ)13N followed by the decay 13N(e+ν)13C This combination of He and H burning reactions in one process occurs in a He-flash convective zone, where there is a plenty of He and 12C, provided that a small amount of H is ingested into it when the upper convective boundary reaches the H-rich envelope layer. In this situation, the reactions 12C(p,γ)13N and 13C(α,n)16O are spatially separated, each taking place at its own favorable conditions, and 13N with a half life of 9.96 min decays into 13C while being carried down by convection with a comparable turnover timescale of ∼ 15 min
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