Impact of a new H/He equation of state on the evolution of massive brown dwarfs. New determination of the hydrogen burning limit

TL;DR

This study examines how a new hydrogen-helium equation of state affects the evolution, cooling, and mass limits of brown dwarfs, leading to better alignment with observations and a revised hydrogen-burning minimum mass.

Contribution

It introduces a new EOS for dense H/He mixtures and demonstrates its impact on brown dwarf evolution and the hydrogen-burning limit, improving model-observation agreement.

Findings

Faster cooling rates and smaller radii for brown dwarfs with the new EOS.

Revised hydrogen-burning minimum mass to 0.075 solar masses.

Enhanced agreement between models and dynamical mass observations.

Abstract

We have explored the impact of the latest equation of state (EOS) for dense hydrogen-helium mixtures (Chabrier \& Debras 2021), which takes into account the interactions between hydrogen and helium species, upon the evolution of very low mass stars and brown dwarfs (BD). These interactions modify the thermodynamic properties of the H/He mixture, notably the entropy, a quantity of prime importance for these fully convective bodies, but also the onset and the development of degeneracy throughout the body. This translates into a faster cooling rate, i.e. cooler isentropes for a given mass and age, and thus larger brown dwarf masses and smaller radii for given effective temperature and luminosity than the models based on previous EOSs. This means that objects of a given mass and age, in the range $M\lesssim 0.1\,\msol$, $\tau\gtrsim 10^8$ yr, will have cooler effective temperatures and fainter luminosities. Confronting these new models with several observationally determined BD dynamical masses, we show that this improves the agreement between evolutionary models and observations and resolves at least part of the observed discrepancies between the properties of dynamical mass determinations and evolutionary models. A noticeable consequence of this improvement of the dense H/He EOS is that it yields a larger H-burning minimum mass, now found to be $0.075\,\msol$ ($78.5\,\mjup$) with the ATMO atmosphere models for solar metallicity. These updated brown dwarf models are made publicly available.