Understanding ultracool atmospheres using benchmark brown dwarfs

(Pinfield, Burningham, Lucas, Jones, Napiwotzki)

The most complete understanding of brown dwarfs and indeed giant planets with ultra-cool temperatures will come from a good understanding of the physics of their atmospheres, and the ability this brings to fit their physical properties (Teff, logg, [M/H]) from observation (e.g. spectroscopy). However, ultracool atmospheres are very complex, with molecular opacities, dust grain condensation, and non-equilibrium chemistry remaining major challenges (as our high resolution spectroscopic studies have shown). To test and guide the models, we populate the parameter-space of physical properties directly, using benchmark brown dwarfs - whose atmospheric and/or physical properties can be inferred in a relatively model free way. We search for benchmark brown dwarfs of several varieties.

• Brown dwarfs in moving group associations and clusters constitute young benchmarks with well constrained ages and uniform (within any group/cluster) composition.
• Widely separated brown dwarf companions in multiple systems can have their properties (distance, age, composition) constrained from studies of the other component stars.
• In particular, wide companions to subgiant stars and high-mass white dwarfs can have ages and/or compositions known to a high degree of accuracy. Wide companions to young stars (e.g. active M dwarfs) can have Teff and logg in the giant planet domain.
• Closer binary systems may be resolved with high resolution imaging and yield dynamical mass measurements.