Pink Dwarfs and the Paths to Stardom: How Brown Dwarfs Pushed Above the Hydrogen Burning Limit Evolve

TL;DR

This paper investigates the evolution of brown dwarfs gaining mass through binary interactions, revealing a luminosity plateau and providing observational predictions for objects near the hydrogen burning limit.

Contribution

It offers a new taxonomy and simulation-based predictions for brown dwarfs crossing the hydrogen burning threshold, highlighting observable evolutionary stages.

Findings

Extended luminosity plateau lasting 100 Myr to Gyr.

Core re-heating timescales determine the luminosity plateau duration.

Potential observational signatures in mass-luminosity diagrams.

Abstract

Brown dwarfs that gain mass through binary interactions may be pushed above the boundary that divides brown dwarfs from low-mass stars: the hydrogen burning limit (HBL). Some of these objects will make their way to the main sequence and may eventually be indistinguishable from ordinary low-mass stars, while others will remain brown dwarf-like, unable to burn hydrogen at a high enough rate to power their surface luminosity. We study the evolution of both types of object to provide a taxonomy and testable observational predictions for these objects depending on their evolutionary path. Using MESA simulations, we find that a subset of the objects that will eventually become stars experience an extended luminosity plateau, where their surface luminosity remains nearly constant on 100 Myr - Gyr timescales. We find that the plateau timescale is set by the amount of energy required to re-heat the cores of these objects to a level sufficient to sustain convection. The timescales required for the cores of these objects to "unfreeze" and arrive at the main sequence is long enough that surveys may be able to find objects in this evolutionary stage. These objects, along with those that never reach the main sequence, occupy a unique space in a mass-luminosity diagram, and would provide a unique constraint on binary mass transfer physics.