My Thesis in 400 Words – Érika Le Bourdais

Érika Le Bourdais, a master’s student at iREx, recently completed her master’s degree at the Université de Montréal. Here she summarizes her research project.

In about five billion years, the Sun will deplete all its resources and shed its outer layers. The remaining core left behind will be what is known as a “white dwarf” star. White dwarfs have roughly half the mass of the Sun contained within the volume of planet Earth. This makes them extremely dense objects! Typically, we observe either hydrogen or helium is the atmosphere of white dwarfs. Metals are often found too, indicating that a planetary system once existed around the white dwarf. The strong gravity of white dwarfs pulls in remnants of these systems, adding them to the star’s atmosphere.

This artist’s illustration depicts a white dwarf star with a surrounding debris disk of material. Photo credit: NOIRLab/NSF/AURA/J. da Silva

Studying the chemical composition of white dwarfs allows us to learn a lot about the building blocks of planetary systems. Indeed, traditional methods for detecting exoplanets can’t tell us about the internal composition of exoplanets. This is why my field is very useful for my colleagues at iREx!

During my master’s degree, I studied in detail the chemical composition of the white dwarf WD 1145+017. This star is unique because it has a debris disk made of asteroids orbiting it. These asteroids create transits—periods when they pass in front of the star—that are up to five times deeper than those caused by exoplanets! The unique shape of the transits indicates the presence of multiple asteroid pieces, some of which have tails like comets. Moreover, the shape of these transits changes over time, indicating that these pieces are disintegrating. A few years ago, the transits disappeared completely. Some astronomers believe that this variation in the disk may be due to a mechanism like the one behind the formation of Saturn’s moons.

My work has been to study the composition of the disk around WD 1145+017 to understand its impact on the observed chemical composition in the star’s atmosphere. We wanted to simulate the disk contamination and then subtract it from the star’s light so that we could obtain the real composition of this white dwarf. I discovered that the presence of the disk could make us detect ten times more metals in the atmosphere than there actually are. This discovery is important because it tells us that the presence of a debris disk can strongly impact our ability to characterize the true abundance of different elements in the star itself. If we have the wrong chemical composition for these stars, we may think that there are much more exotic rocky objects than there actually are and make false assumptions about planetary systems in the Universe.

More information

Érika completed her master’s degree between 2022 and 2024, under the supervision of iREx professor Patrick Dufour. Her thesis will be available soon online.