2025ApJ...994..198D

We extend the Sonora Diamondback brown dwarf evolution models to higher effective temperatures to treat the evolution of younger, higher-mass objects. Due to an upper temperature limit of T_eff = 2400 K in the original Sonora Diamondback model grid, high-mass objects (M ≥ 0.05 M_⊙ = 52.4 M_J) were limited to ages of ≳100 Myr. To include the early evolution of brown dwarfs at T_eff > 2400 K, we use existing and new SPHINX cloud-free model atmosphere calculations of temperature structures of M-type atmospheres. These atmospheres range from T_eff 2000 to 4000 K, log(g) 3.0 to 5.5, and metallicity [M/H] -0.5 to +0.5. This combination of Diamondback and SPHINX atmospheres, with a transition across T_eff 2000-2400 K, allows us to calculate evolution tracks, and infrared photometry and colors, for ages >1 Myr and masses from above the hydrogen-burning minimum mass down to planetary masses. The Hayashi phase of massive brown dwarf evolution (ages < 10-100 Myr) at low surface gravity leads to nearly constant T_eff values, at effective temperatures much lower than would be obtained from simply extrapolating backward from evolution tracks at older ages.