Johanna Vos

Brown dwarfs are objects with masses lying between planets and stars. Although they are generally thought to form like stars, their fundamental properties (mass, age, temperatures)oh are more planet-like. In recent years, a population of young, ‘planetary-mass’ brown dwarfs have been discovered. This young population of objects have remarkably similar spectra and colours to the small population of directly-imaged exoplanets that have been discovered, but are easier to observe since they lack a bright, nearby host-star. Studying the atmospheres of these free-floating exoplanet analogs will reveal insight into the atmospheres of the directly-imaged exoplanets.

We can use photometric variability monitoring to probe atmospheric features in brown dwarf atmospheres such as condensate clouds composed of iron and silicates. In this technique, we measure the brightness of the brown dwarf as it rotates, and detect the presence of these clouds as they rotate in and out of view. In this work, we present photometric variability monitoring using the Spitzer Space Telescope of three isolated, planetary-mass exoplanets analogs. We characterise the variability properties of each target, and place our results in context with the full sample of brown dwarfs with Spitzer variability measurements in the literature. We find three main conclusions:
1. Brown dwarfs spin up over time: Measuring the rotation rates of a sample of young, age-calibrated brown dwarfs allows us to trace angular momentum evolution from 1 Myr to 1 Gyr. We compare the measured rotation rates with predictions set by evolutionary models and find that the rotation rates generally agree with models.
2. Increase in variability for late-Ls: We find a tentative increase in 4.5 um variability for low-gravity, late-L dwarf compared to their higher mass field brown dwarf counterparts. This finding is supports other tentative claims for high-amplitude variability in young, late-L objects in the near-IR.
3. Brown dwarfs viewed equator-on appear redder than the median: We find a 3sigma correlation between the inclination angle and the color anomaly of a sample of 21 brown dwarfs. Brown dwarfs viewed equator-on appear bluer than the median and brown dwarfs viewed pole-on appear bluer than the median. An inclination-dependent color may be explained by thicker clouds accumulating at the equator relative to the poles, as recently predicted by atmospheric dynamical simulations (Tan & Showman 2020).
These results of this study provide important insights into the variability properties of directly-imaged exoplanets such as HR 8799 and β Pic b.