The Sonora Substellar Atmosphere Models. VI. Red Diamondback: Extending Diamondback with SPHINX for Brown Dwarf Early Evolution

Abstract 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.