Numerical Studies of Accretion Flows onto a Neutron Star Engulfed in a Massive Star

TL;DR

This study uses advanced simulations to analyze accretion flows onto neutron stars during common envelope evolution, revealing complex shock structures and force dynamics that challenge traditional models.

Contribution

It introduces a novel multi-layer domain-decomposition simulation strategy and provides new insights into drag forces during neutron star common envelope phases.

Findings

Nested bow shocks are common and alter force dynamics.

Drag force is significantly higher than traditional estimates.

Force direction can reverse depending on envelope conditions.

Abstract

Massive stars commonly form binaries that can evolve into compact systems via common envelope evolution (CEE), a critical but poorly understood phase -- especially when the companion is a neutron star. Understanding the drag force exerted on a neutron star during CEE is a key to the quantitative evaluation of orbital decay, merger timescale, and compactness of the resultant binary. In this paper, we conduct general-relativistic hydrodynamical simulations under a novel strategy of multi-layer domain-decomposition to treat the vast disparity of $10^4$--$10^7$ between the neutron star radius and the accretion radius. Our 10-model survey spans diverse physical conditions that the neutron star encounters in the envelope of a massive star. We find that nested bow shocks with alternating orientations commonly form. This configuration is qualitatively different from those in the conventional picture and results in an enhancement of the drag force by one to two orders of magnitude from what the Bondi--Hoyle--Lyttleton formula predicts. Moreover, the direction of the net force can reverse depending on the envelope conditions, contrary to the standard picture in which the drag always decelerates the companion. These results will serve as a basis for improvements of the drag force prescription in CEE modeling, and have implications for binary evolution theory.