Neutron stars in Palatini R plus alpha R-2 and R plus alpha R-2 + beta Q theories

We study solutions of the stellar structure equations for spherically symmetric objects in modified theories of gravity, where the Einstein-Hilbert Lagrangian is replaced by f(R) = R + alpha R-2 and f(R,Q) = R + alpha R-2 + beta Q, with R being the Ricci scalar curvature, Q = R mu nu R mu nu and R-mu nu the Ricci tensor. We work in the Palatini formalism, where the metric and the connection are assumed to be independent dynamical variables. We focus on stellar solutions in the mass-radius region associated to neutron stars. We illustrate the potential impact of the R-2 and Q terms by studying a range of viable values of alpha and beta. Similarly, we use different equations of state (SLy, FPS, HS(DD2) and HS(TMA)) as a simple way to account for the equation of state uncertainty. Our results show that for certain combinations of the alpha and beta parameters and equation of state, the effect of modifications of general relativity on the properties of stars is sizeable. Therefore, with increasing accuracy in the determination of the equation of state for neutron stars, astrophysical observations may serve as discriminators of modifications of General Relativity.

Automated summary

The study focuses on solutions of stellar structure equations in modified theories of gravity, particularly in the context of neutron stars. By exploring a range of viable values of alpha and beta, the potential impacts of R-2 and Q terms are illustrated. Results suggest that modifications of general relativity can have a significant effect on the properties of stars under certain combinations of parameters and equations of state, indicating that astrophysical observations may serve as discriminators of modifications of General Relativity as the accuracy of determination of neutron star equations of state increases.