The Physics of Mass Transfer in Substellar and Low-mass Binaries

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Abstract Several dozen binary ultracool and brown dwarf (BD) systems have been identified to date. These systems represent valuable probes of star and planet formation at the lowest mass scales. To date, the study of these ultracool binaries has been constrained to the noninteracting case. In this letter, we investigate the dynamics, stability, and evolution of mass transferring ultracool binaries using numerical simulations with accepted equations of state for BDs. We find that there exists a donor mass inversion, above which the donor dwarf is more massive than the accretor, but below which the accretor is more massive than the donor. Below the hydrogen-burning limit, objects with mass ratios of q ∼ 1 are unstable, but slight deviations from this mass ratio are stable at the onset of mass transfer and remain stable throughout extended periods. We compute theoretical mass transfer rates using several angular momentum loss prescriptions and predict lifespans of approximately 0.1–1 Gyr. We predict that all mass-transferring ultracool binaries are tidally locked and possess orbital periods ranging from just under 1 to 3.5 hr. We find that mass transfer proceeds via direct impact onto the accretor, forming a UV or optically bright hotspot on the surface of the accretor.

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