We perform a systematic study of hybrid star configurations using several parametrizations of a relativistic mean-field hadronic EoS and the NJL model for three-flavor quark matter. For the hadronic phase we use the stiff GM1 and TM1 parametrizations, as well as the very stiff NL3 model. In the NJL Lagrangian we include scalar, vector and 't Hooft interactions. The vector coupling constant g v is treated as a free parameter. We also consider that there is a split between the deconfinement and the chiral phase transitions which is controlled by changing the conventional value of the vacuum pressure −Ω 0 in the NJL thermodynamic potential by −(Ω 0 + δΩ 0 ), being δΩ 0 a free parameter. We find that, as we increase the value of δΩ 0 , hybrid stars have a larger maximum mass but are less stable, i.e. hybrid configurations are stable within a smaller range of central densities. For large enough δΩ 0 , stable hybrid configurations are not possible at all. The effect of increasing the coupling constant g v is very similar. We show that stable hybrid configurations with a maximum mass larger than the observed mass of the pulsar PSR J1614-2230 are possible for a large region of the parameter space of g v and δΩ 0 provided the hadronic equation of state contains nucleons only. When the baryon octet is included in the hadronic phase, only a very small region of the parameter space allows to explain the mass of PSR J1614-2230. We compare our results with previous calculations of hybrid stars within the NJL model. We show that it is possible to obtain stable hybrid configurations also in the case δΩ 0 = 0 that corresponds to the conventional NJL model for which the pressure and density vanish at zero temperature and chemical potential. Subject headings: stars: neutron -equation of state -PSR

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In this work we investigate neutron stars (NS) in f(ℛ,T) gravity for the case R+2λ𝒯, ℛ is the Ricci scalar and 𝒯 the trace of the energy-momentum tensor. The hydrostatic equilibrium equations are solved considering realistic equations of state (EsoS). The NS masses and radii obtained are subject to a joint constrain from massive pulsars and the event GW170817. The parameter λ needs to be negative as in previous NS studies, however we found a minimum value for it. The value should be |λ|≲0.02 and the reason for so small value in comparison with previous ones obtained with simpler EsoS is due to the existence of the NS crust. The pressure in theory of gravity depends on the inverse of the sound velocity vs. Since, vs is low in the crust, |λ| need to be very small. We found that the increment in the star mass is less than 1%, much smaller than previous ones obtained not considering the realistic stellar structure, and the star radius cannot become larger, its changes compared to GR is less than 3.6% in all cases. The finding that using several relativistic and non-relativistic models the variation on the NS mass and radius are almost the same for all the EsoS, manifests that our results are insensitive to the high density part of the EsoS. It confirms that stellar mass and radii changes depend only on crust, where the EoS is essentially the same for all the models. The NS crust effect implying very small values of |λ| does not depend on the theory's function chosen, since for any other one the hydrostatic equilibrium equation would always have the dependence 1/vs. Finally, we highlight that our results indicate that conclusions obtained from NS studies done in modified theories of gravity without using realistic EsoS that describe correctly the NS interior can be unreliable.

We propose a density-dependent function for the attractive interaction in the original van der Waals model to correctly describe the flow constraint at the high-density regime of the symmetric nuclear matter. After a generalization to asymmetric nuclear matter, it was also possible to study the stellar matter regime from this new model. The mass-radius relation for neutron stars under β-equilibrium is found to agree with recent X-ray observations. The neutron star masses supported against gravity, obtained from some parametrizations of the model, are in the range of (1.97 − 2.07)M ⊙ , compatible with observational data from the PSR J0348+0432 pulsar. Furthermore, we verify the reliability of the model in predicting tidal deformabilities of the binary system related to the GW170817 neutron star merger event and find a full agreement with the new bounds obtained by the LIGO/Virgo collaboration.

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