Periodic Reporting for period 4 - MULTIPLES (The MULTIPLicity of supErnova progenitorS)
Reporting period: 2023-04-01 to 2024-09-30
With stellar masses in the range of eight to several hundreds of solar masses (Msun), massive stars are among the most important cosmic engines, each individual object strongly impacting its local environment and populations of massive stars driving the evolution of galaxies throughout the history of the Universe. Stars more massive than 15 Msun rarely, if at all, form and live in isolation but rather as part of a binary or higher-order multiple system. Understanding the life cycle of massive multiple systems, from their birth to their death as supernovae and long-duration gamma ray bursts, is one of the most pressing scientific questions in modern astrophysics.
To obtain the key observational breakthroughs needed to revolutionise our understanding of high-mass stars, the MULTIPLES research programme was developed along three themes:
- Investigate the physical processes that set the multiplicity properties of massive stars.
- Establish the multiplicity properties of unevolved massive stars across the entire mass range.
- Identify and uniquely characterize post-interaction products.
The implementation of the MULTIPLES programme involved ambitious time-resolved observational campaigns targeting large populations of massive stars at key stages of their pre-supernova evolution and in different metallicity environments. These campaigns combined state-of-the-art spectroscopy and high-angular resolution techniques with novel multiplicity and atmosphere analysis methods appropriate for multiple systems. The observational constraints that are obtained in this project have implications that extend well beyond the sole domain of stellar astrophysics.
To obtain the key observational breakthroughs needed to revolutionise our understanding of high-mass stars, the MULTIPLES research programme was developed along three themes:
- Investigate the physical processes that set the multiplicity properties of massive stars.
- Establish the multiplicity properties of unevolved massive stars across the entire mass range.
- Identify and uniquely characterize post-interaction products.
The implementation of the MULTIPLES programme involved ambitious time-resolved observational campaigns targeting large populations of massive stars at key stages of their pre-supernova evolution and in different metallicity environments. These campaigns combined state-of-the-art spectroscopy and high-angular resolution techniques with novel multiplicity and atmosphere analysis methods appropriate for multiple systems. The observational constraints that are obtained in this project have implications that extend well beyond the sole domain of stellar astrophysics.
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.
(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.
As part of WP 1 and 2, we have assembled a large data base of high-angular resolution and of spectroscopic observations of massive OB and Wolf-Rayet stars in various environments and evolutionary stages. In WP 3, we have shown that multiplicity of massive stars is already immediately after the stars have emerged from their natal cloud, rooting definitely the origin of the massive star multiplicity to their formation process. In WP4, we have explored the multiplicity properties in various masses and metallicity environments, showing that the multiplicity fraction is always high and that the derived , bias-corrected period and mass-ration distributions are in statistical agreement, suggested (close to) universal multiplicity properties for massive stars across the explore mass (8-60Msun) and metallicity (Zsun to Zsun/5) environments. In WP5, we have investigated the properties of evolved massive stars post-interaction products. We have put the multiplicity properties of WR stars on firm grounds, and have shown that their properties does not seem to match predictions of current binary evolution.
We have unveiled three prototypical objects of rare classes of post-interaction binaries:
- Binary systems formed by an B-type star and a bloated stripped star that is out of thermal equilibrium as a result of their recent accretion of material (Abdul-Masih+2020, Bodensteiner+2020, Shenar+2020, Frost+2022, Hennicker+2022, Villasenor+2023).
- A rejuvenated, magnetic merger product revealed by interferometry, linking magnetism in massive stars to a merger event (Frost+2024).
- The first 2 dormant black with and OB star companion (Mahy+2022, Shenar+2022, Banyard+2023).
The objects have revealed key evolutionary phases, shedding new light on the outcome of mass transfer and the formation of compact objects. The MULTIPLES results progressed the field beyond the state of the art at the time of publication. The discovery of bloated stripped stars was a surprise as this evolutionary phase was expected to be very short. Our work now suggests that the phase might be longer lived due to nuclear shell burning. The focus on dormant black holes was unplanned, but we realised during the project that the assembled observational data base was putting us in a unique position to tackle this project.
We have unveiled three prototypical objects of rare classes of post-interaction binaries:
- Binary systems formed by an B-type star and a bloated stripped star that is out of thermal equilibrium as a result of their recent accretion of material (Abdul-Masih+2020, Bodensteiner+2020, Shenar+2020, Frost+2022, Hennicker+2022, Villasenor+2023).
- A rejuvenated, magnetic merger product revealed by interferometry, linking magnetism in massive stars to a merger event (Frost+2024).
- The first 2 dormant black with and OB star companion (Mahy+2022, Shenar+2022, Banyard+2023).
The objects have revealed key evolutionary phases, shedding new light on the outcome of mass transfer and the formation of compact objects. The MULTIPLES results progressed the field beyond the state of the art at the time of publication. The discovery of bloated stripped stars was a surprise as this evolutionary phase was expected to be very short. Our work now suggests that the phase might be longer lived due to nuclear shell burning. The focus on dormant black holes was unplanned, but we realised during the project that the assembled observational data base was putting us in a unique position to tackle this project.