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

¹ Institute of Theoretical Physics, University of Wroclaw, Max Born Place 9, 50-204 Wroclaw, Poland
² IT and Computing Division, A. Alikhanyan National Laboratory, Alikhanian Brothers Str. 2, Yerevan, 0036, Armenia
³ CONICET, Godoy Cruz 2290, Buenos Aires, Argentina
⁴ Departamento de Física, Comisión Nacional de Energía Atómica, Av. Libertador 8250, (1429) Buenos Aires, Argentina

^* e-mail: david.blaschke@uwr.edu.pl
^** e-mail: alexander.ayriyan@gmail.com
^*** e-mail: mateusz.cierniak@uwr.edu.pl
^**** e-mail: ag.grunfeld@gmail.com
^† e-mail: oleksii.ivanytskyi@uwr.edu.pl
^‡ e-mail: mahboubeh.shahrbafmotlagh@uwr.edu.pl

Published online: 22 December 2022

Abstract

We present a systematic investigation of the possible locations for the special point (SP), a unique feature of hybrid neutron stars in the massradius 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.