Thesis Topic: A Renaissance Study of Open Clusters


Thesis Supervisor: Henri Boffin (personal webpage)



Stars are born in groups, which subsequently disperse, populating the Galactic field (Kroupa 1995a,b; Krause et al. 2020). In some cases, stars might remain gravitationally bound to their siblings, forming star clusters, either globular or open clusters. Open clusters are ideal laboratories to test stellar evolution models as well as binary star models.

The ESA mission Gaia has already led to a revolution in the discovery and our understanding of open clusters (Brown 2021). There is no doubt that this revolution will further unfold in unseen ways after the Gaia DR3 release this 13 June 2022, which brings unvaluable new data on radial velocities, chemical composition, stellar characteristics, and binary properties. This, complemented with current and planned spectroscopic surveys, such as GALAH, Apogee, LAMOST, Rave, 4MOST, Weave, MOONS, etc., as well as the possibility to perform very sophisticated numerical simulations that also include binaries and stellar evolution, is moving the study of star clusters into a completely new regime of unprecedented accuracy. 

With Gaia and these surveys, it is possible to infer if a group of stars not only share the same position in the sky, but also the same proper motions – and radial velocity – thereby forming a gravitationally bound structure, such as an open cluster (Cantat-Gaudin 2022). Using the photometric data in several bands (and, soon, physical parameters), it is then possible to create a colour-magnitude diagram (soon, a full Hertzsprung-Russell diagram) to assess their evolutionary state and derive their masses. Information about binarity can also be gathered, either from Gaia or the complementary surveys, to assess the binarity frequency, the binarity properties, and the real masses of stars and of their companions, thereby also obtaining true stellar mass functions (Boffin et al. 2022; Yalyalieva et al. 2022). Thus, one can now start to make quantitative assessments of the properties of open clusters and look at how they evolve with age, cluster mass, and metallicity. We can also study the binary population, looking in detail at how it affects the cluster’s evolution, as well as to study the population of exotic objects that populate these clusters: blue straggler stars, sub- subgiants, chemically polluted stars, etc (Leiner & Geller 2021).

At the same time, progress in computing power – including the possibility to use GPUs for fast processing – as well as the availability of very sophisticated codes, allows now to simulate in detail the formation and evolution of star clusters and of their population (Wang et al. 2020; Cournoyer-Cloutier et al. 2021, Leveque et al. 2022). We can thus, for the first time ever, make detailed comparisons between theory and observations, with an unprecedented level of details. This is thus the golden age for open cluster studies!