My Thesis in 400 Words: Marylou Fournier-Tondreau

Marylou Fournier-Tondreau, an iREx Masters student, completed her M.Sc. at Université de Montréal this summer. Here, she summarizes her Masters research project.

Marylou Fournier-Tondreau. Credit: Marco Campanozzi, La Presse

During my Master’s degree, I led two research projects to study the atmospheres of two exoplanets, HAT-P-18 b and WASP-52 b, using transit spectroscopy. These two gas giants orbiting very close to their star were observed in transit by the NIRISS instrument onboard the James Webb Space Telescope (JWST). Transit spectroscopy is a commonly used method for indirectly observing the atmosphere of exoplanets. During a ”transit”, or when a planet passes in front of its host star, the light passing through the planet’s atmosphere can be broken down into its different colors (wavelengths), also known as a spectrum. The structure and composition of the planetary atmosphere leave their mark on the spectrum of the light observed. By studying this spectrum, astronomers can determine the temperature of the atmosphere, the presence of clouds and mists, the presence of atoms and molecules (sodium, potassium, water vapor, carbon dioxide, etc.) and their abundances.

However, stars are not uniform sources of light; they have active regions on their surface, such as spots and faculae. For active stars, this can complicate the interpretation of observations by introducing features (fingerprints) into the spectra that overlap with those of exoplanet atmospheres. It is then essential to disentangle the fingerprints originating from the planet from those caused by the host star.

An artist’s depiction of a transiting exoplanet. Credit: ESA/Hubble, NASA, M. Kornmesserr, CC BY 4.0

My MSc involved studying the stellar spots occulted by HAT-P-18 b and WASP-52 b during their transit. Prior to the launch of JWST, these regions were often simply removed from observations, however this can lead to incorrect measurements of transit parameters, and thus, erroneous planetary characteristics. My work has determined each spot’s position, radius, and temperature. I have also considered the effects of magnetic pressure in active regions by introducing new factors when modelling the spots. In addition, the study of HAT-P-18 b’s transit spectrum revealed water vapor, clouds, and carbon dioxide in its atmosphere. The observed spectrum also allowed us to detect the presence of other active regions on the star that had not been occulted by HAT-P-18 b. Our research shows that active regions and their effect on transit spectra need to be considered to better constrain the composition of exoplanet atmospheres.

More information

Marylou completed her MSc at Université de Montréal between 2021 and 2023, under the supervision of iREx Professor David Lafrenière. Her thesis, “Caractérisation d’atmosphère d’exoplanètes par spectroscopie de transmission en présence d’hétérogénéités stellaires : impact et modélisation des régions actives occultées”, is available on Papyrus.