A Strange Star Scenario for the Formation of Eccentric Millisecond Pulsar/Helium White Dwarf Binaries

TL;DR

This paper proposes a novel scenario where phase transitions to strange quark stars in neutron stars within LMXBs can produce eccentric millisecond pulsar/white dwarf binaries, explaining some peculiar systems with unexpected orbital eccentricities.

Contribution

It introduces the strange star formation scenario as a mechanism for eccentric MSP/white dwarf binaries, expanding understanding of their formation beyond traditional models.

Findings

Eccentric MSPs can result from neutron star to strange quark star transitions.

Mass loss during phase transition can account for observed eccentricities.

Scenario may explain the formation of Cir X-1 with an eccentric orbit.

Abstract

According to the recycling scenario, millisecond pulsars (MSPs) have evolved from low-mass X-ray binaries (LMXBs). Their orbits are expected to be circular due to tidal interactions during the binary evolution, as observed in most of the binary MSPs. There are some peculiar systems that do not fit this picture. Three recent examples are PSRs J2234$+$06, J1946$+$3417 and J1950$+$2414, all of which are MSPs in eccentric orbits but with mass functions compatible with expected He white dwarf companions. It has been suggested these MSPs may have formed from delayed accretion-induced collapse of massive white dwarfs, or the eccentricity may be induced by dynamical interaction between the binary and a circumbinary disk. Assuming that the core density of accreting neutron stars in LMXBs may reach the density of quark deconfinement, which can lead to phase transition from neutron stars to strange quark stars, we show that the resultant MSPs are likely to have an eccentric orbit, due to the sudden loss of the gravitational mass of the neutron star during the transition. The eccentricities can be reproduced with a reasonable estimate of the mass loss. This scenario might also account for the formation of the youngest known X-ray binary Cir X$-$1, which also possesses a low-field compact star in an eccentric orbit.