Thesis Topic: The early stages of star cluster formation and evolution


Thesis Supervisor: Giacomo Beccari



The formation and evolution of star clusters has experienced a remarkable regeneration thanks to the successful Gaia mission. The exquisite accuracy
of astrometric measurements offered by Gaia Data Release 2 (DR2) first and very recently the (early) Data Release 3 (E)DR3, coupled with machine learning techniques, allowed the identification and detailed analysis of the stellar populations of hundreds of clusters and associations in the Galaxy (see, e.g., Cantat-Gaudin 2020, A&A, 633, 99).

Specifically, the use of Gaia to identify the members of close-by (<400pc) young (<40Myr) star forming regions has changed our understanding of the physical extent of star formation events. In this context, our group has published in the recent years pioneering results. In Beccari et al. (2020, MNRAS, 491, 2205), we have used Gaia DR2 to study the stellar population in an area of 40deg radius centered on Gamma Vel, a young (<5Myr) association of Pre-Main Sequence stars located around the Wolf–Rayet Star γ2 Vel. Through this study we have identified a 260pc long filamentary structure of coeval stars that connects several 30 Myr old associations and known clusters. This filamentary structure is interpreted as the relict of the formation of stars along a more than 100 pc-long filament.

Similarly, in Jerabkova et al. (2019, MNRAS, 489, 4418) we identified a long (90pc), thin (about 10pc width), co-eval (10Myr) stellar structure in the Orion star-forming region which is likely a relic of star formation along a molecular cloud filament, naming it the Orion relic filament. In both cases, we exclude that such populations originate by the same mechanism responsible to create tidal streams around older clusters. We are instead observing a constellation of young and coeval star clusters and associations few million years after their formation, bridged on a more than 100pc scale by a filament of coeval stars. Such structures are likely the outcome of a mechanism of filamentary star formation in a Giant Molecular Clouds (see also the Stellar Snake; Wang et at. 2021, MNRAS, 513, 503). It is also worth to mention that the detailed study of the stellar population and kinematics of young star-forming regions reveal that most of the close-by regions traditionally though to host simple and coeval populations, in reality show a complex kinematics, indicating that star formation history can last in a single region for more than 10Myr (e.g., Squicciarini et al. 2021, MNRAS, 507, 1381). In this respect, in Beccari et al. (2017, A&A, 604, 22) we discovered three well separated population of Pre-Main Sequence (PMS) stars in the Colour-Magnitude Diagram (CMD) of the Orion Molecular Cluster (ONC). The result was later confirmed using Gaia DR2 in Jerabkova et al. (2019, A&A, 627, 57), where we explain that the star formation history of the ONC, traditionally described trough a single 3Myr old burst of start formation, should instead be described by three discrete episodes, each separated by about a million years. This result is not yet fully accepted in the community. The set of new measurements (binarity, rotation, stellar parameters) available with Gaia DR3 will allow us to give a final word to this problem.

Clearly, these newly discovered structures are the ideal laboratory to study an extraordinary rich and diverse variety of scientific topics. They are superb samples to study the details of early stages of star and cluster formation, the impact of stellar binarity in young populations, the initial mass function of low-mass stars and its evolution with time, the relationship between young stellar populations and Spiral Arms other than bringing the study of filamentary star formation (e.g., Kumar et al. 2022, A&A, 658, 114) to an unpredicted physical scale.

Plan for the PhD

The goal of the PhD project is to first use Gaia to identify the stellar members of the star forming regions in a volume of 500pc from the Sun (e.g. Lupus, Taurus, Upper-Sco, Orion). Moreover, the aim is to extend the work to intermediate age clusters and association (<100Myr) to study the early stages of cluster formation. Once the candidate members are identified via Gaia, multi-band and multi-telescope archival data and from public surveys can be combined with the imnformation obtained with Gaia to e.g. study the mass accretion rates in Pre-Main sequence stars and the impact of the environments (cluster mass, stellar density) on the early stages of stellar evolution hence including the kinematics of the regions.

Overall, the direction in which the project will go will very much depend on the interest of the student.

During the 3 years of Thesis,  the student might have the possibility to collaborate with experts on N-BODY coding (e.g. Prof. Michela Mapelli) to study from a theoretical perspective the early stages of star and cluster formation. Other possible scientific directions the project can take are the spectroscopic or ALMA based follow-up of interesting objects hosting protoplanetary-disks. As mentioned above, this will very much depend on the curiosity and interest of the students. 


Tereza Jerabkova, Henri Boffin, Monika Petr-Gotzens, Carlo Manara (ESO)

Michela Mapelli (UNI-PD, Italy)

Pavel Kroupa (UNI-Bonn, Germany)

Long Wang (School of Physics and Astronomy, Sun Yat-sen University, China)