Irradiated atmospheres of brown dwarfs: providing an insight into exoplanet atmospheres

I will use this fellowship to explore, and ultimately understand how irradiation affects the atmospheres of brown dwarfs.

Brown dwarfs are the missing link between stars and exoplanets. They are the size of Jupiter but may be up to 70 times as massive. However, unlike exoplanets, they form like stars, from the collapse of a massive gas cloud. The first brown dwarf was imaged in 1995, the very same year as the first exoplanet detection, and to date over 1200 have been discovered. Our closest brown dwarf is the Sun's fourth nearest neighbour! Young brown dwarfs have very similar atmospheres to directly imaged exoplanets, and brown dwarfs are often used to tell us what we would expect to see in an exoplanet atmosphere. However, most exoplanets are orbiting very close to their host star, which is much brighter, making it very difficult to determine what is in its atmosphere.

I will study brown dwarfs that are in very close orbits with a hot companion. In these systems, the brown dwarf may complete an orbit in as little as 2 hours. These systems are tidally locked, meaning that the same side of the brown dwarf continually faces the companion. I discovered that this hot side can be up to 500 degrees hotter than the dark side of the brown dwarf, causing winds that transport energy from the hot side to the cold side. It is clear that heating a brown dwarf or exoplanet drastically alters its atmosphere, as the irradiation splits up and creates new molecules.

I will observe these binaries using 4m and 8m telescopes around the world. My observations will monitor the brown dwarf as it orbits its companion, to understand how much brighter the brown dwarf gets as its heated side moves into view. By observing in different wavelengths, I will be able to see how the different layers in the atmosphere respond to the heating. I will also take spectra of the brown dwarf at different points in its orbit. The new James Webb Space Telescope, launched in 2018 is an ideal facility for this work. I will use these spectra to determine what the atmosphere is made of, and which molecules are being created by the irradiation. The creation of new molecules depends very much on how hot the brown dwarf's companion is, so studying as many systems as possible, with host stars of different temperatures is very important.

There are, however, very few of these systems known to date, so as well as studying the known systems, I will discover new ones using a new survey for discovering exoplanets, NGTS, and also wide field surveys. These surveys image the sky at many wavelengths, and by combining data from them I will be able to identify new binaries. I will then obtain more observations of them with world leading telescopes, to measure the masses, and the orbital timescale.

Finally I will work with my collaborators to create models of these systems. These models will be used to explain the processes occurring in the brown dwarf atmosphere as it is heated and will also be used to explain what happens in heated exoplanet atmospheres.