The impact of (n,γ) reaction rate uncertainties of unstable isotopes nearN= 50 on the i-process nucleosynthesis in He-shell flash white dwarfs

Abstract

The anomalous abundances of n-capture elements in the CEMP-r/s stars agree in many instances very well with simulation predictions of intermediate n-density nucleosynthesis, N n ∼ 10 13-10 15 cm −3 , in rapidly-accreting white dwarfs (RAWDs). We have performed Monte-Carlo simulations of this i-process nucleosynthesis in order to determine the impact of (n,γ) reaction rate uncertainties of 164 unstable isotopes, from 131 I to 189 Hf, on the prediction of abundances of 18 elements from Ba to W. The impact study is based on two representative one-zone models with constant values of N n = 3.16 × 10 14 cm −3 and N n = 3.16 × 10 13 cm −3 and on a multi-zone simulation based on a realistic stellar evolution model of He-shell convection entraining H in a RAWD model with [Fe/H] = −2.6. For each of the selected elements, we have identified up to two (n,γ) reactions having the strongest correlations between their rate variations constrained by Hauser-Feshbach computations and the predicted abundances, with the Pearson product-moment correlation coefficients |r P | > 0.15. We find that the possible discrepancies between the predicted and observed abundances of Ba and Pr in the CEMP-r/s star CS31062-050 could be significantly diminished if the rate of the reaction 137 Cs(n,γ) 138 Cs were reduced and the rates of 141 Ba(n,γ) 142 Ba or 141 La(n,γ) 142 La increased. We show that uncertainties of temperature-dependent β-decay rates of the same unstable isotopes have a negligible effect on the predicted abundances and that one-zone Monte-Carlo simulations can be used instead of computationally time-consuming multi-zone Monte-Carlo simulations in reaction rate uncertainty studies if they use comparable values of N n .

The i-process yields of rapidly accreting white dwarfs from multicycle He-shell flash stellar evolution models with mixing parametrizations from 3D hydrodynamics simulations

“…Our i-process nucleosynthesis simulations include ∼ 1000 isotopes and ∼ 15000 reactions. The reaction rates for these simulations are taken from the same list of references as in Denissenkov et al (2018). We adopt an equally spaced 100-zone mass grid for the He shell region by interpolating the stellar structure variables to the new mesh.…”

Nuclear Reactions in Astrophysics: A Review of Useful Probes for Extracting Reaction Rates

“…Charge-exchange reactions are isobaric transitions in which a neutron in the target is exchanged with a proton in the projectile, or vice versa. These reactions can be performed using singlenucleon probes, such as (n, p) or (p, n) reactions, but experimentally it is often advantageous to use composite probes, such as (t, 3 He) or (d, 2 He), or even heavy-ion probes, such as ( 12 C, 12 N) and ( 7 Li, 7 Be). These reactions are mediated by the strong nuclear force via pion exchange but populate the same initial and final states as processes mediated by the weak force and, therefore, can be used as probes in regions where β-decay or β-delayed neutron-emission (β-n) data are unavailable or energetically forbidden (see Figure 5).…”

“…For many years, there has been the idea of an intermediate nucleosynthesis process (i-process) that involves neutron-rich isotopes further away from stability than the s-process but not as exotic as the r-process isotopes. Over the last few years, we have come to understand that a slew of different environments can trigger the i-process, including post-asymptotic giant branch stars, carbonenhanced metal-poor stars, Pb-deficient metal-poor stars, and rapidly accreting white dwarfs (3).…”

Level densities of Ge74,76 from compound nuclear reactions

“…JINA REACLIB [1], BRUSLIB [2] are calculated using the Hauser-Feshbach reaction model [3], which needs nuclear level densities as input. Understanding these inputs is critical, as model calculations have indicated that nuclear reaction rate variations have a marked impact on a variety of phenomena, such as nucleosynthesis via the r-process [4], i-process [5], pprocess [6], νp-process [7], and α-capture in neutron-rich ν-driven winds [8], as well as on the rp-process which powers X-ray bursts on accreting neutron stars [9]. Nuclear level densities are virtually unmeasured for nuclei off the valley of β-stability and Hauser-Feshbach (HF) models employing existing theoretical estimates already find a factor of 3 or more variability in resulting nuclear reaction rates [10,11].…”