Buchdahl's inequality in five dimensional Gauss-Bonnet gravity

Abstract: The Buchdahl limit for static spherically symmetric isotropic stars is generalised to the case of five dimensional Gauss-Bonnet gravity. Our result depends on the sign of the Gauss-Bonnet coupling constant $\alpha$. When $\alpha>0$, we find, unlike in general relativity, that the bound is dependent on the stellar structure, in particular the central energy density. We find that stable stellar structures can exist arbitrarily close to the event horizon. Thus stable stars can exist with extra mass in this theory compared to five dimensional general relativity. For $\alpha<0$ it is found that the Buchdahl bound is more restrictive than the general relativistic case.