Separating expansion and collapse in general fluid models with heat flux
  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.
