Asymmetrical mass ejection from proto-white dwarfs and the formation of eccentric millisecond pulsar binaries

TL;DR

This paper proposes a new model where asymmetrical mass ejection from proto-white dwarfs during thermonuclear flashes imparts kicks that can explain the observed eccentricities in millisecond pulsar binaries, challenging previous theories.

Contribution

It introduces a novel mechanism involving asymmetrical mass ejection from proto-WDs to account for eccentric millisecond pulsar binaries, addressing uncertainties in prior models.

Findings

Simulations show kicks of a few km/s can produce observed eccentricities.

The model aligns with observed eccentricities without requiring circumbinary disks.

It offers an alternative explanation to accretion-induced collapse and other theories.

Abstract

Binary millisecond pulsars (MSPs) are believed to have descended from low-mass X-ray binaries (LMXBs), which have experienced substantial mass transfer and tidal circularization. Therefore, they should have very circular orbits. However, the discovery of several eccentric binary MSPs (with eccentricity $e\sim 0.01-0.1$) challenges this standard picture. Three models have been proposed thus far based on accretion-induced collapse of massive white dwarfs (WDs), neutron star-strange star transition, and formation of circumbinary disks. All of them are subject to various uncertainties, and are not entirely consistent with observations. Here we propose an alternative model taking into account the influence of thermonuclear flashes on proto-WDs. We assume that the flashes lead to asymmetrical mass ejection, which imparts a mild kick on the proto-WDs. By simulating orbital changes of binary MSPs with multiple shell flashes, we show that it is possible to reproduce the observed eccentricities, provided that the kick velocities are around a few kms$^{-1}$.