Special point "trains" in the M-R diagram of hybrid stars

TL;DR

This paper investigates the location of the special point in the mass-radius diagram of hybrid neutron stars using various equations of state, revealing invariant features and bounds related to the star's maximum mass and radius.

Contribution

It introduces the concept of SP trains in the M-R diagram and shows their invariance across different nuclear matter EOSs, highlighting their significance for hybrid star properties.

Findings

SP trains are invariant against changes in nuclear matter EOS.

The SP train related to diquark coupling variation sets a lower bound for maximum mass and radius.

The study demonstrates the impact of vector and diquark couplings on hybrid star features.

Abstract

We present a systematic investigation of the possible locations for the special point (SP), a unique feature of hybrid neutron stars in the mass-radius diagram. The study is performed within the two-phase approach where the high-density (quark matter) phase is described by the covariant nonlocal Nambu--Jona-Lasinio (nlNJL) model equation of state (EOS) which is shown to be equivalent to a constant-sound-speed (CSS) EOS. For the nuclear matter phase around saturation density different relativistic density functional EOSs are used: DD2p00, its excluded-volume modification DD2p40 and the hypernuclear EOS DD2Y-T. In the present contribution we apply the Maxwell construction scheme for the deconfinement transition and demonstrate that a simultaneous variation of the vector and diquark coupling constants results in the occurrence of SP "trains" which are invariant against changing the nuclear matter EOS. We propose that the SP train corresponding to a variation of the diquark coupling at constant vector coupling is special since it serves as a lower bound for the line of maximum masses and accessible radii of massive hybrid stars.