My thesis in 400 words: Dominic Couture

Dominic Couture recently completed his Ph.D. at the Université de Montréal last fall. Here, he summarizes his doctoral research project.

Young stellar associations are sparse, gravitationally unbound groups of stars that formed at the same epoch during a single stellar formation event. Unlike open clusters, which are more readily visible as an overdensity of stars in the sky, young stellar associations are much harder to identify. This is because the member stars, scattered over a large fraction of the celestial sphere, are difficult to distinguish from other stars.

During my thesis, I developed an analysis tool to assess the age of young stellar associations in the solar neighborhood. Obtaining a reliable measure of the age of these stars is of undeniable interest, as a low-mass star close to the Sun, with a well-known age, is a prime target for the search for exoplanets and their study. To do this, I used the traceback method, which involves tracing back the course of member stars in the galaxy and identifying the epoch when the group’s extent in space was minimal. This provides a so-called kinematic age for the cluster, which can be compared with other age estimates, such as those based on stellar evolution models.

My first focus was on one of the closest and best-studied associations known as β Pictoris (βPMG), . To establish its age, I used the positions (astrometric data) and velocities (kinematic data) of the members contained in numerous highly accurate surveys, including those from the third catalog produced with the Gaia space telescope (Gaia DR3, for Data Release 3). My analysis demonstrated that it is essential to minimise the presence of stars that are not members, for example, systems of several stars whose individual members cannot be distinguished. We also need to correct for biases that systematically induce errors on one component of the velocity and a bias due to measurement errors. After testing several ways of performing the analysis, I was able to estimate the kinematic age of the association at around 20.3 million years (Ma), with an uncertainty of 3.4 Ma. This is the first time that a kinematic method has produced a result compatible to previous βPMG age estimates found in the scientific literature.

I then applied my traceback analysis tool to three other associations: Tucana-Horologium (THA), Columba (COL) and Carina (CAR). This enabled me to determine that the best way to assess the size of these associations was to calculate the average length of all the branches linking the different member stars together. This yields results that optimize precision, accuracy and contrast. I thus found kinematic ages of 44.8 Ma for THA and 28.4 Ma for COL and CAR. These results show that it is possible to find a kinematic age compatible with other age assessments for older associations such as THA, as well as for COL and CAR.

In the future, this method could be applied to other nearby young associations, notably with the fourth catalog of the Gaia mission, which will be available in 2026.

Read more

Dominic completed his PhD at the Université de Montréal between 2019 and 2024, under the supervision of Professor René Doyon and Associate Professor Jonathan Gagné of IREx. His thesis, Âge cinématique des associations stellaires jeunes du voisinage solaire, is available (in French) online.