We obtain a second equation by demanding continuity of the stress-energy tensor: ∇ ∇ μ μ T ν ν μ μ = 0 . Observing that ∂ ∂ t ρ ρ = ∂ ∂ t P = 0 (since the configuration is assumed to be static) and that ∂ ∂ ϕ ϕ P = ∂ ∂ θ θ P = 0 (since the configuration is also isotropic), we obtain in particular