De begeleiders van zware sterren: van sterren tot bruine dwergen

Massive stars are born with initial masses exceeding approximately eight solar masses. Although rare and short-lived, typically only surviving a few million years, these objects have a substantial impact on their surroundings through powerful stellar winds, intense ultraviolet radiation, and chemical enrichment. At the end of their lives, massive stars may explode as supernovae and form a neutron star or a black hole. Understanding, the birth, evolution and death of massive stars is a fundamental goal of modern astronomy. Despite their importance, the formation of massive stars remains an open question. A key distinction between low- and high-mass star formation is the prevalence of multiplicity. Massive stars form almost exclusively in binary or higher-order multiple systems. Constraining their multiplicity properties is therefore essential for testing theories of massive and multiple star formation. Understanding how such systems evolve requires studying massive stars across a range of ages and environments. Yet current observations remain incomplete for systems in which the companion has a mass smaller than one tenth of that of the central star, due to instrumental contrast limits. The presence of low-mass companions (less than one solar mass) around massive stars thus remains uncertain. The intense UV radiation from massive stars significantly affects their surroundings, raising the question of whether low-mass companions can form and survive in such harsh environments. High-contrast imaging with the Spectro-Polarimetric High-contrast Exoplanet REsearch instrument at the Very Large Telescope (VLT/SPHERE) now offers a way to address these questions by probing multiplicity down to extremely low companion masses. Using SPHERE data for over 100 massive stars, we examine how multiplicity varies with both age and environment, and we account for observational biases to enable consistent comparisons between datasets and with theoretical predictions. We find a multiplicity fraction for massive stars close to 100% by studying the multiplicity properties of a sample of 19 OB stars in the Scorpius OB1 association. Additionally, we identify four low-mass candidate companions with a mass below 0.5 solar mass. Proper motion measurements of three of these candidate companions indicate that two are co-moving with their central star, strongly suggesting that they are gravitationally bound to their central star. We further report the discovery of two massive stars hosting brown dwarf candidate companions in Scorpius OB1 and in M17, respectively. Although the substellar nature of these objects requires confirmation, these results indicate that low-mass companions may indeed exist around massive stars, though their formation pathways remain uncertain. Finally, we develop a Python-based algorithm to correct for observation biases in high-contrast imaging surveys and apply it to six different datasets that vary in age or stellar number density. Our results suggest that massive stars host more low-mass companions in low-density environments, and they provide support for a migration-driven formation scenario for binaries in M17. In conclusion, high-contrast imaging has opened a new window on the low-mass companion population of massive stars. In the future, combining these observational results with theoretical predictions will be essential to improve our understanding about the formation of massive stars and their companions.