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

We explored the impact of the latest equation of state (EOS) for dense hydrogen-helium mixtures, which takes into account the interactions between hydrogen and helium species during the evolution of very low-mass stars and brown dwarfs (BDs). 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, that is, cooler isentropes for a given mass and age, and thus larger BD masses and smaller radii for a 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 less than or similar to 0.1 M-circle dot, tau greater than or similar to 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 discrepancy 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 M-circle dot (78.5M(Jup)) with the ATMO atmosphere models for solar metallicity. These updated BD models are made publicly available.

Automated summary

We investigated the impact of a new equation of state (EOS) for dense hydrogen-helium mixtures on the evolution of low-mass stars and brown dwarfs. The EOS considers interactions between hydrogen and helium species, affecting the thermodynamic properties of the mixture and leading to cooler temperatures and fainter luminosities for objects within a given mass and age range. Comparing these new models with observations of dynamical masses of brown dwarfs improves the agreement between evolutionary models and observations and resolves part of the observed discrepancy. Furthermore, the dense EOS yields a larger H-burning minimum mass.