Separating expansion and collapse in general fluid models with heat flux

Abstract: In this paper we consider spherically symmetric general fluids with heat flux, motivated by causal thermodynamics, and give the appropriate set of conditions that define separating shells defining the divide between expansion and collapse. To do so we add the new requirement that heat flux and its evolution vanish at the separating surface. We extend previous works with a fully nonlinear analysis in the 1+3 splitting, and present gauge-invariant results. The definition of the separating surface is inspired by the conservation of the Misner-Sharp mass, and is obtained by generalizing the Tolman-Oppenheimer-Volkoff equilibrium and turnaround conditions. We emphasize the nonlocal character of these conditions as found in previous works and discuss connections to the phenomena of spacetime cracking and thermal peeling.