The Irradiated Atmospheres Of Brown Dwarf Companions To White Dwarf Stars

Currently there are 1000’s of known exoplanets, many of which are hot-jupiters (Jupiter-like planets close in to their host star), but very few have had their atmospheres characterised. The study of close non-interacting white dwarf - brown dwarf binaries offers a unique opportunity to observe a planet-like atmosphere without the observational challenges faced when investigating exo-atmospheres. In this work I investigate two systems: WD0137-349 and NLTT5306. The former displays irradiation induced metal emission lines in a substellar atmosphere. I investigated the lines strengths of the H and Ca II line and found these to vary with orbital phase and almost completely disappear on the dark-side of the brown dwarf; implying that heat redistribution around this tidally locked companion is likely to be poor. replication of this result will be necessary in future planetary atmosphere models. NLTT6306 is almost identical to WD0137-349 in component mass and orbital period, the main difference being the age and temperature of the white dwarf primary. This system however, is very different. NLTT5306 displays H emission and Na I absorption associated with the white dwarf which indicates accretion is happening. However, there are no high energy emissions, Zeeman splitting, or evidence of a disc. An X-ray non-detection gives a maximum accretion rate of 2 x 10−15M⊙ yr−1 which would only require a magnetic field of 0.45 kG to prevent a disc from forming whilst still too low produce detectable Zeeman splitting. I have concluded that the observations can be explained by a light wind from the brown dwarf being magnetically funnelled onto the white dwarf surface. Finally, I have analysed Gemini near-infrared spectra from 8 white dwarfs with photometrically confirmed infrared excesses. I compared the spectra to composite models and I was able to spectroscopically confirm 3 new white dwarf - brown dwarf binaries.