Deep crustal heating for realistic compositions of thermonuclear ashes

TL;DR

This study presents the first thermodynamically consistent calculations of deep crustal heating in accreting neutron stars, considering realistic thermonuclear ash compositions and neutron hydrostatic/diffusion equilibrium, providing new insights into their thermal evolution.

Contribution

It introduces a novel, thermodynamically consistent method for calculating crustal heating that accounts for neutron hydrostatic/diffusion equilibrium in realistic ash compositions.

Findings

Lower limit on crustal heating energy Q > 0.13-0.2 MeV per baryon

Neutron hydrostatic/diffusion equilibrium significantly affects heating calculations

Realistic ash compositions influence the energy release estimates

Abstract

The deep crustal heating, associated with exothermal nuclear reactions, is believed to be a key parameter for describing the thermal evolution of accreting neutron stars. In this paper, we present the first thermodynamically consistent calculations of the crustal heating for realistic compositions of thermonuclear ashes. In contrast to previous studies based on the traditional approach, we account for neutron hydrostatic/diffusion (nHD) equilibrium condition imposed by superfluidity of neutrons in a major part of the inner crust and rapid diffusion in the remaining part of the inner crust. We apply a simplified reaction network to model nuclear evolution of various multi-component thermonuclear burning ashes (superburst, KEPLER, and extreme rp-process ashes) in the outer crust and calculate the deep crustal heating energy release Q, parametrized by the pressure at the outer-inner crust interface, P_{oi}. Using the general thermodynamic arguments, we set a lower limit on Q, Q>0.13-0.2 MeV per baryon (an actual value depends on the ash composition and the employed mass model).