An analysis of the TZ Fornacis binary system

Abstract

Context. TZ Fornacis (TZ For) is an evolved detached binary system that is difficult to model and interpret, but very useful for testing stellar evolution theory and physics. Aims. We aim to search for solutions that are self-consistent and to determine the necessary stellar physics input. We also check solutions found previously for their internal consistency and for reproducibility. Methods. We use both a single and a binary stellar evolution code, and take into account all known system properties. We determine the physical stellar parameters by imposing that the models match the known radii for identical stellar ages. The evolution has to be consistent with a binary system in classical Roche geometry. Results. We obtained two different solutions to model TZ For successfully. Both depend on avoiding a long evolution on the first giant branch and imply a sufficiently large convective core on the main sequence. TZ For can be modelled consistently as a detached binary system by invoking either a substantial amount of core overshooting or a tidally enhanced wind mass loss along the red giant branch. An evolution with Roche-lobe overflow can definitely be excluded. Conclusions. A comparison of our results with previous studies also reveals that in addition to uncertainties associated with the input physics, the modelling of overshooting by different algorithms can have a strong impact.