Atmosphere Models of Highly Irradiated Brown Dwarfs

While highly irradiated brown dwarfs are rare, the handful of known systems provide a unique lens into irradiated atmospheres. Ultra-short-period brown dwarfs around white dwarfs can reach atmospheric temperatures hotter than many stars. Such objects are similar to ultra-hot Jupiters, yet are subject to even more intense UV irradiation, allowing us to test irradiated atmosphere models and reveal new insight into substellar atmospheres. We present 1D PHOENIX atmosphere models and synthetic spectra, as well as retrievals with the PETRA framework, of two highly irradiated brown dwarfs around white dwarfs. Crucially, these models self-consistently include the effects of the intense UV irradiation on the atmosphere. We will show that we can explain the observed presence of a variety of emission lines in these brown dwarfs through the an irradiation-induced temperature inversion, not unlike those recently seen in several ultra-hot Jupiters.