The Role of Strong Gravity and the Nuclear Equation of State on Neutron-Star Common-Envelope Accretion

TL;DR

This paper investigates how strong gravity effects and nuclear physics influence neutron star growth and spin during common-envelope evolution, affecting binary properties and merger rates.

Contribution

It models the impact of relativistic mass deficit and nuclear equation of state on neutron star accretion and spin-up in common-envelope phases, highlighting their astrophysical significance.

Findings

Neutron star mass growth is suppressed by approximately 15-30%.

More compact neutron stars spin-up faster but gain less gravitational mass.

Strong gravity and nuclear microphysics influence neutron star binary populations.

Abstract

Common-envelope evolution is important in the formation of neutron star binaries within the isolated binary formation channel. As a neutron star inspirals within the envelope of a primary massive star, it accretes and spins up. Because neutron stars are in the strong-gravity regime, they have a substantial relativistic mass deficit, i.e., their gravitational mass is less than their baryonic mass. This effect causes some fraction of the accreted baryonic mass to convert into neutron star binding energy. The relativistic mass deficit also depends on the nuclear equation of state, since more compact neutron stars will have larger binding energies. We model the mass growth and spin-up of neutron stars inspiraling within common-envelope environments and quantify how different initial binary conditions and hadronic equations of state affect the post-common-envelope neutron star's mass and spin. From these models, we find that neutron star mass growth is suppressed by $\approx 15-30\%$. We also find that for a given amount of accreted baryonic mass, more compact neutron stars will spin-up faster while gaining less gravitational mass, and vice versa. This work demonstrates that a neutron star's strong gravity and nuclear microphysics plays a role in neutron-star-common-envelope evolution, in addition to the macroscopic astrophysics of the envelope. Strong gravity and the nuclear equation of state may thus affect both the population properties of neutron star binaries and the cosmic double neutron star merger rate.