Relativistic polytropic spheres with electric charge: Compact stars, compactness and mass bounds, and quasiblack hole configurations

Abstract

We study the static stellar equilibrium configurations ofuncharged and charged spheres composed by a relativistic polytropic fluid, and compare with those of spheres composed by a non-relativistic polytropic fluid, the later case already being studied in a previous work [J. D. Arba\~nil, P. S. Lemos, V. T. Zanchin, Phys. Rev. D \textbf{88}, 084023 (2013)]. In the relativistic fluid case, a relativistic polytropic equation of state, $p=\omega\delta^{\gamma}$, is assumedd. Here, $\delta=\rho-p/(\gamma-1)$, with $\delta$ and $\rho$ being the rest mass density and the energy density, respectively, and $\omega$ and $\gamma$ are respectively the polytropic constant and the polytropic exponent. We assume that the charge density $\rho_e$ is proportional to the energy density $\rho$, $\rho_e = \alpha\, \rho$, with $\alpha$ being a constant such that $0\leq |\alpha|\leq 1$. Some properties of the charged spheres such as mass, total electric charge, radius, redshift, and the speed of sound are analyzed. The dependence of such properties with the polytropic exponent is also investigated. In addition, some limits that arise in general relativity, such as the Chandrasekhar limit, the Oppenheimer-Volkoff limit, the Buchdahl bound and the Buchdahl-Andr\'easson bound, i.e., the Buchdahl bound for the electric case, are studied. As in a charged non-relativistic polytropic sphere, the charged relativistic polytropic sphere with $\gamma\to\infty$ and $\alpha \to 1$ saturates the Buchdahl-Andr\'easson bound, thus indicating that it reaches the quasiblack hole configuration. We show by means of numerical analysis that, as expected, the major differences between the two cases appear in the high energy density region.