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

TL;DR

This paper explores the dynamics, stability, and evolution of mass transferring ultracool binary systems, revealing conditions for stability, mass transfer rates, and observational signatures through numerical simulations.

Contribution

It provides the first detailed numerical analysis of mass transfer processes in ultracool and brown dwarf binaries, including stability criteria and predicted observational features.

Findings

Existence of donor mass inversion in ultracool binaries

Mass transfer rates estimated at approximately 100 million years lifespan

All such binaries are predicted to be tidally locked with short orbital periods

Abstract

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.