The Cosmic Waltz of Ultracool Dwarfs express
Two dim stars whirl so close together that one siphons material off the other, like a spoon delicately skimming foam from broth. Astronomers have for the first time calculated the fate of such pairs — ultracool and brown dwarfs, the coldest and dimmest inhabitants of the galaxy. Previously, it was thought that mass exchange between them was impossible: they were too fragile. But simulations showed that if the masses are nearly equal, the system is unstable, while with a slight difference, it can stably exist for a very long time.
When matter flows over, it does not form a disk but crashes directly onto the surface of the second star. At the impact point, a bright spot appears — in ultraviolet or even visible light. Such a pair completes a full orbit in one to three and a half hours, and both stars always face each other with the same side, like dancers in a slow waltz. The system is destined to live for about one hundred million years — short by cosmic standards, but for observers on Earth, it is an eternity.
Thus, the coldest stars in the galaxy turn out to be capable of turbulent relationships. By studying them, we better understand how objects on the boundary between planets and stars are born and die.
Original abstract
as written by the authors arXiv:2603.17040
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.
Ultracool dwarfs are so faint that they can only be seen with infrared telescopes, even though their masses are comparable to a star's.