The GSF Instability and Turbulence do not Account for the Relatively Low Rotation Rate of Pulsars

TL;DR

This study investigates the role of GSF instability and turbulence in rotating stars, concluding that they do not significantly influence the low rotation rates observed in pulsars.

Contribution

The paper derives new expressions for GSF instability considering thermal transport and horizontal turbulence, applying them to pre-supernova stellar models.

Findings

GSF instability persists in late stellar stages.

Localized GSF transport can exceed shear instability near convective zones.

Insufficient mixing occurs before supernova, not affecting pulsar rotation rates.

Abstract

The aim of this paper is to examine the effects of the horizontal turbulence in differentially rotating stars on the GSF instability and apply our results to pre-supernova models. For this purpose we derive the expression for the GSF instability with account of the thermal transport and smoothing of the mu-gradient by the horizontal turbulence. We apply the new expressions in numerical models of a 20 solar mass star. We show that if N^2_{Omega} < 0 the Rayleigh-Taylor instability cannot be killed by the stabilizing thermal and mu-gradients, so that the GSF instability is always there and we derive the corresponding diffusion coefficient. The GSF instability grows towards the very latest stages of stellar evolution. Close to the deep convective zones in pre-supernova stages, the transport coefficient of elements and angular momentum by the GSF instability can very locally be larger than the shear instability and even as large as the thermal diffusivity. However the zones over which the GSF instability is acting are extremely narrow and there is not enough time left before the supernova explosion for a significant mixing to occur. Thus, even when the inhibiting effects of the mu-gradient are reduced by the horizontal turbulence, the GSF instability remains insignificant for the evolution. We conclude that the GSF instability in pre-supernova stages cannot be held responsible for the relatively low rotation rate of pulsars compared to the predictions of rotating star models.