Since the beginning of the project, the MULTIPLES team has focused on three scientific questions:
(1) what the origin of massive star multiplicity is and of the pairing mechanism;
(2) what are the multiplicity properties of massive star populations for which it has been less well established, including B-type stars, Wolf-Rayet (WR) stars, and sub-solar metallicity environments, and
(3) what is the role of binary evolution in the formation of evolved objects such as WRs, Be stars, stripped stars and merger products.
To help answering these questions, the MULTIPLES team has collected and analysed new high-angular-resolution and spectroscopic observations and developed and implemented novel analysis methods and bias correction techniques. It has further established strong synergies with other projects and researchers at KU Leuven and beyond.
We have showed that multiplicity occur early in the life of massive stars, indicating that its origin is most likely rooted in the star formation process (Bordier+2022). Yet, the properties of distant low-mass companions, expected to be produced by disk fragmentation do not match predictions. We indeed not a low number of solar-mass companions and an abundance of very-low mass stars, and even sub-stellar companions. (Reggiani+2021, Pauwels+2023,2024)
We brought the multiplicity properties of O- (Lanthermann+2023) and B-type stars (Banyard+2022, Villasenor+2021) on firmer ground. We show that about half of the B stars have companions on a less than 10-year orbit and that their period distribution is compatible with that derived at higher masses and at LMC metallicities. This points towards a joint pairing mechanism across the massive star regime. We have measured the massive binary fraction at SMC metallicity, showing that massive binaries are important in such environment too , and strongly suggesting that they are also important in the distant Universe (Sana+, subm).
We have obtained some of the most precise mass measurements of stars more massive than 50 solar masses, providing new high-quality data to confront evolutionary models (Fabry+2021). We have investigated the that most Wolf-Rayet stars are not in (long-period) binaries and that binary evolution is not necessarily required to explain Wolf-Rayet stars in the Large Magellanic Clouds (Shenar+2021).
Some of the most significant achievements of the projects are:
- The discovery of dormant black-holes in massive binaries (Shenar+2022, Mahy+2022)
- The discovery of bloated stripped stars (Shenar+2020, Frost+2022)
- Finding a link between binary interaction and magnetism (Frost+2024)
- Finding that massive star multiplicity properties seem universal to first order (Villasenor+2021, Banyard+2022), also in a low-metallicity environment representative of high-redshift Universe (Sana+subm, Villasenor+ subm.)
The MULTIPLES programme further provided a strong contribution to provide the community with a legacy data set of massive star multi-epoch spectroscopy in the Small Magellanic Clouds
The scientific success of the MULTIPLES project is evidenced by the more than 65 refereed publications in international journals, including high-impact journals such as Nature and Nature Astronomy. At this moment the combined citation count of the MULTIPLES publications is ~2000, showing the impact of the work on the field and the contribution to the state of the art.
