Supermassive neutron stars in axion F(R) gravity

Abstract

ABSTRACT We investigated realistic neutron stars in axion R2 gravity. The coupling between curvature and axion field ϕ is assumed in the simple form ∼R2ϕ. For the axion mass in the range ma ∼ 10−11–10−10 eV the solitonic core within neutron star and corresponding halo with size ∼100 km can exist. Therefore the effective contribution of R2 term grows inside the star and it leads to change of star parameters (namely, mass, and radius). We obtained the increase of star mass independent from central density for wide range of masses. Therefore, maximal possible mass for given equation of state grows. At the same time, the star radius increases not so considerably in comparison with GR. Hence, our model may predict possible existence of supermassive compact stars with masses $M\sim 2.2\!-\!2.3\, \mathrm{M}_\odot$ and radii Rs ∼ 11 km for realistic equation of state (we considered APR equation of state). In general relativity one can obtain neutron stars with such characteristics only for unrealistic, extremely stiff equations of state. Note that this increase of mass occurs due to change of solution for scalar curvature outside the star. In GR curvature drops to zero on star surface where ρ = p = 0. In the model underconsideration the scalar curvature dumps more slowly in comparison with vacuum R2 gravity due to axion ‘galo’ around the star.