Olivia Lim

As part of her PhD thesis, Olivia focuses on TRAPPIST-1. This planetary system consists of a red dwarf star and seven orbiting exoplanets. These seven companions are very peculiar because they transit their host star, that is, they pass in front of the star as seen from Earth. These seven planets have sizes similar to that of Earth and three of them are in the habitable zone, which is the region in the system where liquid water can exist at the surface of a planet. TRAPPIST-1 is therefore an ideal target to search for signatures of life outside the solar system with the next generation of observational facilities such as the James Webb space telescope.

One of the properties of TRAPPIST-1 that Olivia studies is the mass of the planets. When a planet orbits its star, it induces the same orbital movement on the star, but with a weaker amplitude, as the planet is less massive than the star. We can detect this subtle movement of the star by measuring its radial velocity in time, that is, the component of its velocity that is parallel to the line of sight. With this measurement, we can infer the mass of the planet. We can apply the same method to each of the seven planets and thus determine the seven masses. Combined with the known planet radii, these masses give us the density of the planets, which tells us about their composition: are they mostly rocky planets? are they rather covered by an envelope of water? However if the star exhibits heterogeneities such as dark spots on its surface, these can also induce a signal in radial velocity that can be misinterpreted as planetary signal. This makes it more complicated to disentangle the information coming from a planet from the one coming from the star itself. This is why we need to model the stellar signal to discriminate between planet and star signal to correctly measure the planetary masses.

Olivia also studies the geometric configuration of TRAPPIST-1 to better understand its formation history. More specifically, the angle between the orbital plane of the planets and the equatorial plane of the star can tell us if, and if appropriate, how, the planets and the star have interacted in the past. This angle is also measured with the radial velocity of the star, using the Rossiter-McLaughlin effect.

Olivia is simultaneously participating in the reduction of data from the infrared spectropolarimeter (SPIRou) at the Canada-France-Hawaii Telescope. Olivia works on the impact of persistence on radial velocity measurements. Persistence is a phenomenon similar to the ghost image we see after staring at a sunset for too long: when SPIRou looks at two targets successively, the observation of the second object is contaminated by the residual image of the first on the detector. Since radial velocities are important for a variety of astrophysical fields, we must understand how persistence affects these measurements obtained from SPIRou.

Supervisor

René Doyon