Supermassive neutron stars in axion F(R) gravity

We investigated realistic neutron stars in axion R-2 gravity. The coupling between curvature and axion field phi is assumed in the simple form similar to R-2 phi. For the axion mass in the range m(a) similar to 10(-11)-10(-10) eV the solitonic core within neutron star and corresponding halo with size similar to 100 km can exist. Therefore the effective contribution of R-2 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 similar to 2.2-2.3 M-circle dot and radii R-s similar to 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 rho = p = 0. In the model underconsideration the scalar curvature dumps more slowly in comparison with vacuum R-2 gravity due to axion 'galo' around the star.