Hybrid Stars with Different Amount of Quark Matter

Abstract

In a previous work, we explored the possibility of neutron stars with large quark cores. Through Bayesian inference, we obtained large sets of equations of state imposing constraints regarding the properties of nuclear matter, mass-radius data from NICER, the GW170817 data from Virgo-LIGO collaboration, perturbative QCD and causality. We also imposed that the maximum mass neutron star (NS$\text{max}$) must have a large quark core. In this work, we obtain a new set of equations of state (SmallQ set) with the same constraints as before. However, we relaxed the last constraint, imposing only that NS$\text{max}$ must have quark core, regardless of its size. We compare the results with the set from our previous paper constrained by NICER and GW170817 observations NJL-GW. For both sets, we use the Relativistic Mean Field model with nonlinear meson interactions for the hadron phase and the Nambu--Jona-Lasinio model with four- and eight-quark interaction terms for the quark phase. The SmallQ set has higher values of phase transition density, with 3 -- 4.5 $ρ_0$, compared to the NJL-GW, with 2 -- 2.5 $ρ_0$. The new set describes NSs with smaller radius and smaller tidal deformability, and is more compatible with NICER and GW170817 data. A difference in the quark matter composition is observed, with a larger quark strange fraction occurring in the SmallQ set. Within this set, $2 M_\odot$ NSs may contain little to no quark matter. It is shown that there is a non negligible fraction of EOS whose mass-radius curve presents a positive slope for 1.8 $M_\odot$ stars, while for nucleonic stars this fraction is negligible and for hyperonic stars the slope is always negative.