Understanding how stars form and evolve is one of the most fascinating challenges in astronomy. A key piece of this puzzle is the stellar multiplicity—the frequency, separations, and mass ratios of stars that form in pairs or higher-order groups. Massive stars, which make up only 0.01% of all formed stars, play a pivotal role in shaping galaxies and the universe. Understanding their life cycle—90% of which unfolds alongside at least one companion—is therefore of paramount importance. Yet, they are particularly difficult to study since they are heavily bloated in their dusty envelopes and are born in distant, dense, and young star clusters. These observational challenges leave significant gaps in our understanding of how their multiplicity evolves over time.
To help fill this gap, our team conducted high-resolution observations to explore the properties of stars that have just formed or are still in the process of forming, within four massive, near-primordial clusters. We identify potential binary or multiple star systems, study how their properties—such as separation and mass ratios—vary within and between clusters. In this framework, multiplicity serves as a tool to trace the initial conditions that drive the formation of massive stars in multiple systems.
To study the primordial multiplicity properties, the clusters must have similar chronological ages with slight variations and be young enough to ensure that significant dynamical interactions have not yet occurred. This increases the likelihood of observing systems in a near-primordial multiplicity state. The selected clusters—Hourglass Nebula, RCW 108, DBS 113, and DBS 121—are observable in the near-infrared and submillimeter wavelengths due to their significant dust emission. These clusters are located at a distance of 1.5 ± 0.3 kpc on average and can be classified by age. Based on this cluster classification, we trace the evolution of multiplicity and companion fraction between 1.0 Myr and 2.8 Myr.
Our preliminary results are derived from NACO K-band data, with Chinmaya, a PhD student in our team, leading this aspect of the project. The figure shows one of the fields of DBS121 for which we show the identified sources, hence potential companions per bin of separation. We find that multiplicity properties evolve with cluster age, as the other clusters show a gradually increasing trend in the number of stars, thereby indicating a possible increase in companion fraction with cluster age. To extend this study, we recently included VVV CL100 (7.5 Myr) to determine whether this trend persists in older clusters.
Emma, a postdoc in our team, successfully led an ALMA proposal to enhance this analysis. Data acquisition is now underway, promising exciting new discoveries in the evolution of multiplicity properties in massive star-forming clusters. Stay tuned!