Compositionally-driven convection in the oceans of accreting neutron stars

TL;DR

This paper explores how chemical separation and convection driven by buoyancy in neutron star oceans lead to significant light element enrichment and heating, impacting models of superburst ignition.

Contribution

It introduces a model of compositionally-driven convection in neutron star oceans, showing how light element retention causes mixing and heating, which was not previously characterized.

Findings

Oxygen abundance in the ocean exceeds 10% despite low production.

Convective heat flux is approximately 0.2 MeV per nucleon.

Enrichment and heating influence superburst ignition models.

Abstract

We discuss the effect of chemical separation as matter freezes at the base of the ocean of an accreting neutron star, and argue that the retention of light elements in the liquid acts as a source of buoyancy that drives a slow but continual mixing of the ocean, enriching it substantially in light elements, and leading to a relatively uniform composition with depth. We first consider the timescales associated with different processes that can redistribute elements in the ocean, including convection, sedimentation, crystallization, and diffusion. We then calculate the steady state structure of the ocean of a neutron star for an illustrative model in which the accreted hydrogen and helium burns to produce a mixture of O and Se. Even though the H/He burning produces only 2% oxygen by mass, the steady state ocean has an oxygen abundance more than ten times larger, almost 40% by mass. Furthermore, we show that the convective motions transport heat inwards, with a flux of ~ 0.2 MeV per nucleon for an O-Se ocean, heating the ocean and steepening the outwards temperature gradient. The enrichment of light elements and heating of the ocean due to compositionally-driven convection likely have important implications for carbon ignition models of superbursts.