Maximum Baryon Masses for Static Neutron Stars in $f(R)$ Gravity

Abstract

We investigate the upper mass limit predictions of the baryonic mass for static neutron stars in the context of $f(R)$ gravity. We use the most popular $f(R)$ gravity model, namely the $R^2$ gravity, and calculate the maximum baryon mass of static neutron stars adopting several realistic equations of state and one ideal equation of state, namely that of causal limit. Our motivation is based on the fact that neutron stars with baryon masses larger than the maximum mass for static neutron star configurations inevitably collapse to black holes. Thus with our analysis, we want further to enlighten the predictions for the maximum baryon masses of static neutron stars in $R^2$ gravity, which, in turn, further strengthens our understanding of the mysterious mass-gap region. As we show, the baryon masses of most of the equations of states studied in this paper, lie in the lower limits of the mass-gap region $M\sim 2.5-5 M_{\odot}$, but intriguingly enough, the highest value of the maximum baryon masses we found is of the order of $M\sim 3 M_{\odot}$. This upper mass limit also appears as a maximum static neutron star gravitational mass limit in other contexts. Combining the two results which refer to baryon and gravitational masses, we point out that the gravitational mass of static neutron stars cannot be larger than three solar masses, while based on maximum baryon masses results of the present work, we can conspicuously state that it is highly likely the lower mass limits of astrophysical black holes in the range of $M\sim 2.5-3 M_{\odot}$. This, in turn, implies that maximum neutron star masses in the context of $R^2$ gravity are likely to be in the lower limits of the range of $M\sim 2.4-3 M_{\odot}$. Hence our work further supports the General Relativity claim that neutron stars cannot have gravitational masses larger than $3$$M_{\odot}$.