HR6819: a puffed-up stripped star system challenging stable mass transfer theory

Context. HR6819 is the first system with a puffed-up low-mass stripped star + a classical Be star whose nature has been confirmed by optical interferometry. The system exhibits the most extreme mass ratio (15.7 ± 1.1), the lowest stripped star mass (0.270 ± 0.056 M⊙), and one of the shortest orbital periods (40.3266 ± 0.0016 days) compared to similar observed binaries. As a post mass transfer candidate, HR6819 offers a unique opportunity to test the physics of binary interaction, especially the fraction of mass accreted by the Be progenitor (the efficiency of the mass transfer) required to produce the system’s extreme mass ratio. Aims. This work aims to reconstruct the possible evolutionary history of HR6819 in the context of stable mass transfer via Roche lobe overflow. We want to explore how the tight constraints on the system’s total mass, mass ratio and orbital period are limiting the range of possible progenitors of the system. Methods. Based on analytical expectations for the orbital evolution, we build grids of MESA simulations designed to match the present-day orbital period and mass ratio of the system, with different mass transfer efficiencies from fully conservative to 50% efficient. Results. We show that evolution via stable mass transfer cannot explain the combined extreme current mass ratio and tight orbital period of the system. There is a limit on how extreme the post-mass transfer mass ratio of the progenitor binary can be at a fixed detachment period, and this limit is dependent on the efficiency of the mass transfer episode: the less efficient the mass transfer episode, the less extreme the mass ratio at detachment. Even in the case of fully conservative mass transfer, the most extreme mass ratio we can produce with binary evolution simulations is q ∼ 11.5 at P ∼ 40 days, which is significantly below the observed value. We also show that the reported luminosities for each component significantly exceed the value expected from their mass. In particular, based on simple stripped star models, we find that the luminosity of the bloated stripped star requires a star with a mass of ∼0.7 M⊙, which is over twice the measured value. Conclusions. Our work shows that the post-interaction properties of HR6819, especially its extreme mass ratio and orbital period, cannot be produced by stable mass transfer under standard assumptions.