PRObes of Gravitational-wave progENITORs

Project Information

PROGENITOR

Grant agreement ID: 101024605

DOI

10.3030/101024605

Project closed

EC signature date 22 March 2021

Start date 1 October 2021

End date 30 September 2023

Funded under

• EXCELLENT SCIENCE - Marie Skłodowska-Curie Actions

Total cost

€ 175 572,48

EU contribution

€ 175 572,48

Coordinated by

UNIVERSITEIT VAN AMSTERDAM
Netherlands

Periodic Reporting for period 1 - PROGENITOR (PRObes of Gravitational-wave progENITORs)

Reporting period: 2021-10-01 to 2023-09-30

Summary of the context and overall objectives of the project

Massive stars - stars born with masses exceeding eight times that of our Sun - are related to a plethora of phenomena in modern astronomy, such as transients and core-collapse supernovae, the Early Universe, and gravitational-wave astrophysics.
PROGENITOR had the overarching goal of advancing our understanding of massive stars by analysing observations of selected samples of massive stars in our Galaxy and a nearby galaxy called the "Large Magellanic Cloud".
Specifically, the research had four main fundamental objectives:
- RO1: Establish whether the most massive stars known are in binary or multiple stellar systems
- RO2: Substantiate the evolutionary pathway of merging pairs of black holes, as observed by gravitational-wave facilities.
- RO3: Establish the multiplicity of black-hole progenitors in the Local Group
- RO4: Boost our theories of massive-star evolution

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

The project has achieved most of its objectives and milestones for the period, with relatively minor deviations.

Scientific objectives:

• RO1: Establish the multiplicity of the most massive stars – fully accomplished.
This objective entailed the analysis of novel spectroscopic data acquired with the Hubble Space Telescope (HST), which monitored the most massive stars known: R136 a1, a2, a3, and c in the Large Magellanic Cloud (LMC). The analysis advanced as planned, with the main result being that no companions were found for these objects, with the exception of R136 c. We concluded that the upper-mass limit still remains well above 200 solar masses. Our findings were published in a peer-reviewed journal (Shenar et al. 2023, A&A, 679, 36)
• RO2: Substantiate the evolutionary pathway of black-hole mergers – fully accomplished.
This objective included the analysis of the shortest-period Wolf-Rayet binaries in the LMC, thought to comprise of two Wolf-Rayet stars. This project was offered as a MSc thesis, which was successfully conducted by Sancho Luitjen. The student provided a full characterisation of the companions, measured their masses and physical properties. The work is published as a MSc thesis: https://scripties.uba.uva.nl/search?id=record_49924 . A corresponding publication in a peer-reviewed journal is under preparation.
In addition, the project included the analysis of spectroscopic monitoring of massive stars in the LMC to uncover black holes orbiting them. The project included the discovery of the first dormant black hole in the LMC, VFTS 243. The results were published in peer-reviewed journals: Shenar et al. 2022, A&A, 665, 148; Shenar et al. 2022, Nature Astronomy, 6, 1085.
A related project led simultaneously focused on a peculiar carbon and nitrogen-rich WR binary, HD 45166, in the Galaxy, whose properties were so far at odds with our understanding of stellar winds. We discovered that the WR star in this binary has the strongest magnetic field ever detected for a massive star, and that it is the first WR star with a measured magnetic field. The results received much media coverage, and were published in the prestigious Science journal (Shenar et al. 2023, Science, 381, 761)
• RO3: Establish the multiplicity of black-hole progenitors in the Local Group – fully accomplished.
This project included the analysis of spectroscopic monitoring of the complete population of carbon-rich Wolf-Rayet stars (WC) in the LMC using novel X-SHOOTER/VLT data. The project was offered as a MSc thesis, which was led by Freek Temming. The student successfully derived the binary fraction of the sample, and the results are published as a MSc thesis: https://scripties.uba.uva.nl/search?id=record_53809 . A corresponding publication in a peer-reviewed journal is under preparation.
• RO4: Boost our theories of massive-star evolution – partly accomplished.
Inputs from deliverables related to RO1-RO3 were included in studies led by theorists regarding Wolf-Rayet binaries (e.g. Sen \& Shenar, 2023, A&A, 672, 198) and black-hole formation (e.g. Rastello \& Shenar, 2023, MNRAS, 526, 740). Pending work still includes a full evolutionary modelling of the shortest Wolf-Rayet binaries in the LMC (RO2), and population synthesis that accounts for the binary fraction of LMC WC stars (RO3).

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

I describe the progress beyond the state-of-the-art & impact per work package (WP):

• WP1: This WP included the analysis of spectroscopy obtained with the STIS instrument of the HST (PI: Shenar), which monitored the most massive stars known – R136 a1, a2, a3, and c in the LMC. Overall, in addition to one archival epoch, three epochs were secured during 2020 – 2021 with HST.
o main scientific achievements: first constraints on multiplicity of most massive stars
o contribution to the state of the art: revised constraints on upper-mass limit

• WP2: This WP included the analysis of novel UVES/VLT data (PI: Shenar) for the shortest-period Wolf-Rayet binaries in the LMC, BAT99 32, which was thought to comprise of two Wolf-Rayet stars. In addition, the project included the analysis of spectroscopic monitoring of massive stars in the LMC to uncover black holes orbiting them. The analysis included an implementation of “spectral disentangling” on a population of single-lined spectroscopic binaries to determine the nature of their hidden companions. These results were published in Shenar et al. 2022, A&A, 665, 148. Remarkably, we managed to uncover the first dormant black hole in the LMC, a result which won a lot of media attention and was published Shenar et al. 2022, Nature Astronomy, 6, 1085.
o main scientific achievements: Unveiling the nature of the shortest-period Wolf-Rayet binary in the LMC, including accurate masses; first discovery of an extragalactic dormant black hole, implying that some massive stars do not explode as supernova and do not suffer natal kicks.
o contribution to the state of the art: Unprecedented constraints on core-collapse into black holes, and unveiling of the nature of the shortest-period Wolf-Rayet star in the LMC


• WP3: This WP included the analysis of spectroscopic monitoring of the complete population of carbon-rich LMC WC stars using novel X-SHOOTER/VLT data. A related project led simultaneously focused on a peculiar carbon and nitrogen-rich WR binary, HD 45166, in the Galaxy, whose properties were so far at odds with our understanding of stellar winds. We discovered that the WR star in this binary has the strongest magnetic field ever detected for a massive star, and that it is the first WR star with a measured magnetic field. The results received much media coverage, and were published in the prestigious Science journal (Shenar et al. 2023, Science, 381, 761)
o main scientific achievements: First derivation of the binary fraction of LMC WC stars, and the discovery of the first magnetic WR star.
o contribution to the state of the art: The LMC WC binary fraction strongly constraints binary evolution, and the magnetic WR star revealed a new astrophysical phenomenon, not previously known.

• WP4: On-going collaborations with theorists were led to integrate the deliverables of WPs1-3 in models of stellar evolution. This includes models related to Wolf-Rayet binaries (e.g. Sen \& Shenar, 2023, A&A, 672, 198) and black-hole formation (e.g. Rastello \& Shenar, 2023, MNRAS, 526, 740).

VFTS 243, the first X-ray quiet black hole uncovered outside our Milky Way Galaxy (ESO/L. CALÇADA)