Nickel isotopes in stellar matter

TL;DR

This paper presents a microscopic pn-QRPA calculation of Gamow-Teller strength distributions and electron capture rates for nickel isotopes, revealing significant differences from shell model results with implications for stellar evolution modeling.

Contribution

It introduces an improved pn-QRPA approach for calculating GT strength functions and electron capture rates on nickel isotopes, highlighting the failure of Brink's hypothesis and the importance of excited states.

Findings

Excited state GT distributions differ significantly from ground states.

Electron capture rates on $^{57,59}$Ni are much higher than shell model predictions.

Excited states contribute dominantly to electron capture during silicon burning.

Abstract

Isotopes of nickel play a key role during the silicon burning phase up to the presupernova phase of massive stars. Electron capture rates on these nickel isotopes are also important during the phase of core contraction. I present here the microscopic calculation of ground and excited states Gamow-Teller (GT) strength distributions for key nickel isotopes. The calculation is performed within the frame-work of pn-QRPA model. A judicious choice of model parameters, specially of the Gamow-Teller strength parameters and the deformation parameter, resulted in a much improved calculation of GT strength functions. The excited state GT distributions are much different from the corresponding ground-state distributions resulting in a failure of the Brink's hypothesis. The electron capture and positron decay rates on nickel isotopes are also calculated within the framework of pn-QRPA model relevant to the presupernova evolution of massive stars. The electron capture rates on odd-A isotopes of nickel are shown to have dominant contributions from parent excited states during as early as silicon burning phases. Comparison is being made with the large scale shell model calculation. During the silicon burning phases of massive stars the electron capture rates on $^{57,59}$Ni are around an order of magnitude bigger than shell model rates and can bear consequences for core-collapse simulators.