Neutron stars in f(ℛ,T) gravity using realistic equations of state in the light of massive pulsars and GW170817

Abstract

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.

Neutron stars in $$f(\mathtt {R,L_m})$$ gravity with realistic equations of state: joint-constrains with GW170817, massive pulsars, and the PSR J0030+0451 mass-radius from NICER data

“…In our analyses we are going to follow the same methodology used in Ref. [39]; we use only the EoS that yields a maximum mass near 2.0 M considering GR; the EoS leading to NS mass-radius need to be within/close to the region delimited by the LIGO-VIRGO observation [52,53]. We choose a set of EoS considering pure nuclear matter and one EoS for hybrid matter (with unconfined quarks).…”

“…The full description of each of these EoS is given in §3 of Ref. [39]. The set of parametrization: WFF1, APR4, SLy and MPA1 were constrained in the analysis of the gravitational wave event GW170817.…”

“…Further generalizations of f (R) gravity were developed. The f (R, T) is a well-known case, proposed by Harko et al [36], it consists of a theory where the gravitational action is an arbitrary function of the Ricci scalar and also of the trace of the energy-momentum tensor, T. The theory has been widely applied to compact stars [37][38][39][40], see §2. [41] for a plenty of them.…”

“…Notwithstanding, in most cases, the researchers considered a simple barotropic equation of state (EoS) describing the matter inside these objects, leading to unreliable results, as reported in Ref. [39]. Using a set of fundamental nuclear matter EoS based on effective models of nuclear interactions and comparing the results with the gravitational-wave observations, as well as with massive pulsars in a joint constrain, the authors claim that the increment in the star mass is less than 1%.…”

Charged compact star in f(R, T) gravity in Tolman–Kuchowicz spacetime

“…In fact, f (R, T ) gravity was phenomenologically introduced by Harko et al [8]. In this connection, we want to mention that recently the anisotropic charged and uncharged models in modified f (R, T ) theory gravity have been studied in [9][10][11][12][13]. Pretel et al [14] examined the static structure configurations and radial stability of compact stars within the context of f (R, T ) gravity.…”