Dr Sarah Casewell and Professor Graham Wynn
Brown dwarfs are often thought of as failed stars. They form like stars, but never burn hydrogen into helium, and so are degenerate objects the size of Jupiter with masses of up to 70 times that of Jupiter and atmospheres dominated by dust clouds and molecules such as methane and carbon monoxide. Brown dwarfs are rarely found in close orbits around main sequence stars – a phenomenon known as the brown dwarf desert. Those that are found here are rare, and difficult to study as the main sequence star is much larger and brighter than the brown dwarf companion.
In order to understand brown dwarfs in these systems more deeply, we can look at their evolved forms - white dwarf-brown dwarf binaries. In these systems the main sequence star has evolved into a giant and engulfed the brown dwarf which has spiralled inwards inside the giant’s envelope. The envelope has then been ejected, leaving the brown dwarf orbiting the white dwarf remnant in a close orbit of a few hours.
The proximity of the brown dwarf means it is tidally locked to the white dwarf and has a heated dayside and colder nightside – a situation we see in hot Jupiter exoplanets. Indeed, these systems can be used as proxies for heated exoplanets as their Jupiter-like atmospheres get heated in the same way. However, these systems are very rare, as only 8 are known. In order to explore the effects of heating on the brown dwarf atmosphere we need to find more systems with a wider range of white dwarf temperatures and more diversity in brown dwarf atmospheres.
In this PhD you will conduct a search for more white dwarf-brown dwarf systems using multi-waveband all sky surveys, and use imaging and spectroscopy from a wide range of telescopes to characterise and investigate the systems more fully. At the end of your PhD you will have discovered some of the rarest systems in the universe and investigated their atmospheres, potentially discovering unique cloud systems, differences in day and night side temperatures of over 500 K and evenauroral-like emission caused by irradiation.
1. The first sub-70 min non-interacting WD–BD system: EPIC212235321, Casewell et al., 2018,MNRAS: http://dx.doi.org/10.1093/mnras/sty245
2. The direct detection of the irradiated brown dwarf in the white dwarf-brown dwarf binary SDSS J141126.20+200911.1: Casewell et al., 2018, MNRAS: http://dx.doi.org/10.1093/mnras/sty2599
3. Emission lines in the atmosphere of the irradiated brown dwarf WD0137−349B: Longstaff, Casewell et al., 2017, MNRAS: http://dx.doi.org/10.1093/mnras/stx1786