Structure of neutron stars in R-squared gravity

TL;DR

This paper investigates how R-squared gravity modifies neutron star structures, revealing potential for heavier stars and unique interior profiles, using numerical methods and simple equations of state.

Contribution

It provides a numerical analysis of neutron star structure in R-squared gravity, highlighting the impact of second-order derivatives of the EoS on interior profiles and mass-radius relations.

Findings

Neutron stars can be heavier in R-squared gravity than in GR.

Interior mass profiles can decrease with radius due to modified gravity effects.

Profiles correlate with the second derivative of the EoS approximation.

Abstract

The effects implied for the structure of compact objects by the modification of General Relativity produced by the generalization of the Lagrangian density to the form f(R)=R+\alpha R^2, where R is the Ricci curvature scalar, have been recently explored. It seems likely that this squared-gravity may allow heavier Neutron Stars (NSs) than GR. In addition, these objects can be useful to constrain free parameters of modified-gravity theories. The differences between alternative gravity theories is enhanced in the strong gravitational regime. In this regime, because of the complexity of the field equations, perturbative methods become a good choice to treat the problem. Following previous works in the field, we performed a numerical integration of the structure equations that describe NSs in f(R)-gravity, recovering their mass-radius relations, but focusing on particular features that arise from this approach in the profiles of the NS interior. We show that these profiles run in correlation with the second-order derivative of the analytic approximation to the Equation of State (EoS), which leads to regions where the enclosed mass decreases with the radius in a counter-intuitive way. We reproduce all computations with a simple polytropic EoS to separate zeroth-order modified gravity effects.