Impact of a new H/He equation of state on the evolution of massive brown dwarfs

New determination of the hydrogen burning limit

¹ École normale supérieure de Lyon, CRAL, CNRS, UMR 5574, 69364 Lyon Cedex 07, France
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² Physics & Astronomy Dpt, University of Exeter, Exeter EX4 4QL, UK
³ Institute for Astronomy, University of Hawaii at Manoa, Honolulu, HI 96822, USA
⁴ IRAP, Université de Toulouse, CNRS UMR 5277, UPS, Toulouse, France

Received: 21 April 2022
Accepted: 19 November 2022

Abstract

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 ≲ 0.1 M_⊙, τ ≳ 10⁸ 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_⊙ (78.5 M_Jup) with the ATMO atmosphere models for solar metallicity. These updated BD models are made publicly available.