Characterising weather and rotation on substellar worlds

Abstract

Photometric variability monitoring is sensitive to atmospheric inhomogeneities as they rotate in and out of view, and is a key probe of atmospheric features in brown dwarfs and giant exoplanets. The population of young, low-gravity brown dwarfs are an excellent test bed for characterising the gravity dependence of brown dwarf atmospheres while also providing a critical analogue to the population of directly-imaged planets. This thesis aims to expand our current understanding of the variability of brown dwarfs with a focus on the role of gravity on variability properties. In Chapter 2 I present the detection of photometric variability in the L7 planetary-mass object PSO 318.5-22, as well as subsequent multi-wavelength ground-based and space-based monitoring. I use the followup observations to measure the rotational period of PSO 318.5-22 and to investigate the horizontal and vertical structure of its atmosphere. In Chapter 3 I supplement the existing brown dwarf variability data in the literature with new rotational velocities to reveal relations between inclination angle, variability amplitude and colour. These new relations probe the latitudinal cloud structure of brown dwarfs for the first time and will inform future searches for variability on both free-floating exoplanets and directly-imaged companions. In Chapter 4 I present Spitzer monitoring of the three lowest-mass members of the AB Doradus moving group. I report mid-IR variability in two late-L exoplanet analogues, W0047 and 2M2244 and place upper limits on the variability of the T5.5 planetary-mass object SDSS 1110. I measure the rotational periods and inclination angles of W0047 and 2M2244 and find that they are consistent with the relations reported in Chapter 3 for the field dwarf sample. Finally, in Chapter 5 I present the first large survey for photometric variability in young low-gravity brown dwarfs. Theory and observations have shown that gravity plays an important role in the atmospheric properties of L and T types objects. Surface gravity significantly affects the height at which condensate clouds form in the atmosphere (Marley et al., 2012) and thus we can expect that gravity will have an effect on variability. I detect variability in 6 low-gravity brown dwarfs, 4 of which are reported in this thesis for the first time. Focusing on the L0-L8/5 objects in the survey, I find a variability occurrence rate of 30+16/-8 %. I reanalyse the results of Radigan (2014) and find a variability occurrence rate of 11+13/-4 % for the field dwarf sample. This is the first quantitative indication that the young objects are more likely to be variable than their higher mass counterparts. The work presented in this thesis has provided crucial insight into the role of gravity on variability properties, and acts as a key pathfinder for future studies of variability on directly-imaged planets.