High-mass twin stars with a multipolytrope equation of state

We show that in the three-polytrope model of Hebeler et al. [Astrophys. J. 773, 11 (2013)] for the neutron star equation of state (EoS) at supersaturation densities a third family of compact stars can be obtained which confirms the possibility of high-mass twin stars that have coincident masses $M_1=M_2\approx 2M_{\odot }$ and significantly different radii $|R_1-R_2|>2–3$ km. We show that the causality constraint puts severe limitations on the maximum mass of the third family sequence which can be relaxed when this scheme is extended to four polytropes thus mimicking a realistic high-density matter EoS.

Figure 1

EoS [(a)—$P$ vs. $\varepsilon$; (b) - $c_s^2$ vs. $\varepsilon$] and sequences of compact stars [(c)—$M$ vs. $R$; (d) - $M_B$ vs. $R$] for sets 1–4 of Table 1. The EoS have the same onset density and density jump of the phase transition, but different stiffness of the high density (quark matter) phase. The plus symbols denote values for the maximum mass configurations.

Figure 2

High mass twins internal energy density profile. The dashed curve corresponds to the pure hadronic star whose radius exceeds the hybrid star by about 3 km.

Figure 3

“Phase diagram” of hybrid star sequences following Ref. [5] for parameter sets 1–4 of this work. For details, see text.

Figure 4

All multi-polytrope EoS for sets 1–4 from Table 1 fall within the region given in Hebeler et al. [27] for the case supporting a $1.97M_{\odot }$ NS (grey shaded region). These EoS share the hadronic branch EoS, the onset density and jump in energy density at the transition but vary in the stiffness of the high-density phase. The plus symbols denote the pressure and energy density values at the center of the compact star configuration with the maximum mass.