Project FOSSO I: Fates of the Known BDs in MS-BD Binaries

TL;DR

This work forward-models the fate of ~200 known MS–BD binaries as their hosts evolve to white dwarfs, using COMPAS to track mass loss, tides, and CE evolution. It predicts a pronounced WD–BD period gap (roughly 1–1000 days) resulting from divergent CE and non-CE pathways, and shows that CE efficiency $\alpha_{\rm CE}$ crucially shapes the surviving close WD–BD and merged outcomes. The simulations reproduce several observed WD–BD systems and predict an unseen population of wide WD–BD binaries near $\sim$15 AU (periods ~100 years), as well as a subset of BD–CVs arising from post-CE evolution; these findings constrain CE physics and tidal interactions and guide future searches via microlensing and high-contrast imaging. Overall, the study links substellar companion evolution to binary-epoch outcomes, offering testable predictions for next-generation observatories and informing the broader understanding of BD formation and binary evolution.

Abstract

Context. Understanding the survival and orbital evolution of brown dwarf (BD) companions during the post-main-sequence (MS) evolution of their host stars is increasingly important, especially with recent discoveries of many substellar companions around white dwarfs (WDs). Aims. We investigate the long-term evolution and final outcomes of BDs orbiting low-mass MS stars as these evolve into WDs. By comparing forward-modeling populations with observed WD-BD binaries, we test evolutionary models and predict the existence of yet-undetected systems. Methods. We employ the COMPAS binary population synthesis code 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 on the period-mass diagram, located at orbital periods of $\sim$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. This work predicts that many WD-BD systems remain undetected, motivating targeted searches with microlensing and high-contrast imaging techniques using next-generation large telescopes.

Figures (7)

• Figure 1: Mass–period distribution of the observed MS--BD binary systems used in this study. The color scale indicates the mass of the host MS star. The top and right histograms with kernel density estimate (KDE) show the distributions of the orbital period and BD mass, respectively. The grey dashed lines show the mass range of BDs, while the green dashed line represents the BD desert transition mass of $42.5~M_{\rm Jup}$ in Ma14.
• Figure 2: Same as Figure \ref{['fig:ob_MSBD_nofate']}, all the markers represent the initial MS--BD systems, while different markers indicating the different fates. The top and right KDE plots represent the distributions of different fates: grey, blue and orange correspond to merged system, close WD--BD and wide WD--BD system, respectively.
• Figure 3: Period-mass distribution for the future WD--BD system of the observed MS--BD binaries, calculated with (top panel) and without (bottom panel) tidal effect. The stars mark the observed closed and wide WD--BD binaries listed in Table \ref{['tab:ob_WDBD']}. The gray area shows the period gap around 1--1000 day. The dashed line represents the minimum period for the binary to evolve to CVs.
• Figure 4: Fates of known BDs around MS. Blue and pink dots represent the initial and final separations of the survived MS--BD binaries. The size of the dots indicates the mass of BDs. The red dashed circle shows the average of the minimum separations for the binaries to stay detached.
• Figure 5: The CDFs of the mass ratio $q=M_{\rm BD}/M_{\rm WD}$. The left panel shows the mass ratio distribution of close and wide simulated samples, respectively. While the right panel shows the distribution of the observed samples.