Compact stars in the braneworld: a new branch of stellar configurations with arbitrarily large mass

Abstract

We study the properties of compact stars in the Randall-Sundrum II type braneworld model. To this end, we solve the braneworld generalization of the stellar structure equations for a static fluid distribution with spherical symmetry considering that the spacetime outside the star is described by a Schwarzschild metric. First, the stellar structure equations are integrated employing the so called causal limit equation of state (EOS), which is constructed using a well established EOS at densities below a fiducial density, and the causal EOS $P= \rho$ above it. It is a standard procedure in general relativistic stellar structure calculations to use such EOS for obtaining a limit in the mass radius diagram, known as causal limit, above which no stellar configurations are possible if the EOS fulfills that the sound velocity is smaller than the speed of light. We find that the equilibrium solutions in the braneworld model can violate the general relativistic causal limit and, for sufficiently large mass they approach asymptotically to the Schwarzschild limit $M = 2 R$. Then, we investigate the properties of hadronic and strange quark stars using two typical EOSs. For masses below $\sim 1.5 - 2 M_{\odot}$, the mass versus radius curves show the typical behavior found within the frame of General Relativity. However, we also find a new branch of stellar configurations that can violate the general relativistic causal limit and that in principle may have an arbitrarily large mass. The stars belonging to this new branch are supported against collapse by the nonlocal effects of the bulk on the brane. We also show that these stars are always stable under small radial perturbations. These results support the idea that traces of extra-dimensions might be found in astrophysics, specifically through the analysis of masses and radii of compact objects.