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

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Main Authors: Whitebook, Samuel, Fuller, Jim, Burdge, Kevin, Marsh, Thomas R., Mawet, Dimitri, Prince, Thomas
Format: Preprint
Published: 2026
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author Whitebook, Samuel
Fuller, Jim
Burdge, Kevin
Marsh, Thomas R.
Mawet, Dimitri
Prince, Thomas
author_facet Whitebook, Samuel
Fuller, Jim
Burdge, Kevin
Marsh, Thomas R.
Mawet, Dimitri
Prince, Thomas
contents Several dozen binary ultracool and brown dwarf 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 non-interacting case. In this paper, we investigate the dynamics, stability, and evolution of mass transferring ultracool binaries using numerical simulations with accepted equations of state for brown dwarfs. 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 $q \sim 1$ 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 $\sim 100$ Myrs. We predict that all mass transferring ultracool binaries are tidally locked and possess orbital periods ranging from just under $1$ hour to $3.5$ hours. 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.
format Preprint
id arxiv_https___arxiv_org_abs_2603_17040
institution arXiv
publishDate 2026
record_format arxiv
spellingShingle The Physics of Mass Transfer in Substellar and Low-Mass Binaries
Whitebook, Samuel
Fuller, Jim
Burdge, Kevin
Marsh, Thomas R.
Mawet, Dimitri
Prince, Thomas
Solar and Stellar Astrophysics
Several dozen binary ultracool and brown dwarf 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 non-interacting case. In this paper, we investigate the dynamics, stability, and evolution of mass transferring ultracool binaries using numerical simulations with accepted equations of state for brown dwarfs. 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 $q \sim 1$ 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 $\sim 100$ Myrs. We predict that all mass transferring ultracool binaries are tidally locked and possess orbital periods ranging from just under $1$ hour to $3.5$ hours. 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.
title The Physics of Mass Transfer in Substellar and Low-Mass Binaries
topic Solar and Stellar Astrophysics
url https://arxiv.org/abs/2603.17040