Fates of the sub-stellar objects (FOSSO)

Context. Understanding the survival and orbital evolution of brown dwarf (BD) companions during the post-main-sequence (MS) evolution of their host stars has become increasingly important, especially following recent discoveries of substellar companions around white dwarfs (WDs). Aims. We investigated the long-term evolution and final outcomes of BDs orbiting low-mass MS stars as these evolve into WDs. By comparing forward-modelled populations with observed WD–BD binaries, we tested evolutionary models and predicted the existence of yet-undetected systems. Methods. We employed the code Compact Object Mergers: Population Astrophysics and Statistics (COMPAS) to evolve observed MS–BD systems through the post-MS phases of their host stars into the WD stage, tracking orbital changes driven by mass loss, tides, and common-envelope (CE) evolution. Results. Our simulations reproduce a period gap in the distribution of detached WD–BD binaries, consistent with observations. We also identify a boundary separating detached and semi-detached systems in the period-mass diagram at orbital periods of ∼1−2 hours, depending on the BD mass. Conclusions. We predict that a subset of currently known MS–BD binaries will survive post-MS evolution and emerge as detached WD–BD systems, while others will undergo CE evolution and potentially form cataclysmic variables with BD donors. Our results reproduce the observed period gap in WD–BD binaries and provide quantitative predictions for the role of CE efficiency in shaping their distribution. These findings indicate that many WD–BD systems remain undetected, motivating targeted searches using microlensing and high-contrast imaging techniques with next-generation large telescopes.