Neutron drip transition in accreting and nonaccreting neutron star crusts

TL;DR

This paper investigates the neutron drip transition in neutron star crusts, deriving analytical expressions and performing numerical studies for different compositions, highlighting differences between accreting and nonaccreting stars.

Contribution

It provides new analytical formulas for neutron drip density and pressure considering accreting conditions, improving upon previous mean-nucleus approximations.

Findings

Neutron drip occurs at lower densities than previously predicted.

Derived general analytical expressions for neutron drip conditions.

Numerical results for various initial compositions from X-ray burst ashes.

Abstract

The neutron-drip transition in the dense matter constituting the interior of neutron stars generally refers to the appearance of unbound neutrons as the matter density reaches some threshold density $\rho_\textrm{drip}$. This transition has been mainly studied under the cold catalyzed matter hypothesis. However, this assumption is unrealistic for accreting neutron stars. After examining the physical processes that are thought to be allowed in both accreting and nonaccreting neutron stars, suitable conditions for the onset of neutron drip are derived and general analytical expressions for the neutron drip density and pressure are obtained. Moreover, we show that the neutron-drip transition occurs at lower density and pressure than those predicted within the mean-nucleus approximation. This transition is studied numerically for various initial composition of the ashes from X-ray bursts and superbursts using microscopic nuclear mass models.