Massive stars are thought to form by accretion, during the gravitational collapse of dense cores in large interstellar clouds. Their properties at birth, such as their mass and angular momentum, rely on the complex hydrodynamics of the collapse.

Due to their high luminosity, massive stars exert a strong radiative feedback on the surrounding gas and dust during their formation. In particular, their ultraviolet radiation field usually ionises large regions in their vicinity, which might disrupt the accretion process. However, the ionising flux depends significantly on the accretion process. [Haemmerlé et al. 2016, A&A 585 A65; Haemmerlé & Peters 2016, MNRAS 458 3299; Haemmerlé et al. 2019, A&A 624 A137; Meyer et al. 2019, MNRAS 484 2482; Meyer et al. 2019, MNRAS 487 4473; Jaura et al. 2022, MNRAS 512 116]

By angular momentum conservation, accretion is expected to proceed through a disc. Mechanisms like magnetic fields, viscosity, or gravitational torques are required in order to remove angular momentum from the central regions, otherwise the star would rotate so fast that the centrifugal force would overcome gravity and destroy the star. For the gas to be accreted by the star, these mechanisms must be efficient enough to remove more than 2/3 of the angular momentum from the inner disc. [Haemmerlé et al. 2017, A&A 602 A17]

Supermassive stars: the most massive stars in Universe's history?

The recent discovery of quasars at high redshifts, powered by supermassive black holes of millions to billions solar masses, challenges our understanding of the early Universe. The accumulation of such masses in a compact object in less than a billion years requires extreme conditions. The most promising scenario for the formation of these black holes is the direct collapse of interstellar matter into a supermassive star, and the subsequent collapse of the star via the general-relativistic instability. [Haemmerlé et al. 2020, SSR 216 48; Haemmerlé 2021, A&A 647 A83; Haemmerlé 2020, A&A 644 A154; Haemmerlé 2021, A&A 650 A204; Haemmerlé et al. 2021, A&A 652 L7; Zwick et al. 2023, MNRAS 518 2076; Haemmerlé 2024, A&A 689 A202]

 

In this scenario, the supermassive star evolves under accretion at rates >0.1 solar mass per year. As a result of rapid accretion, the star remains a red supergiant, with negligible ionising feedback, which might facilitate detection by the James Webb Space Telescope. [Haemmerlé et al. 2018, MNRAS 474 2757; Haemmerlé et al. 2019, A&A 632 L2; Surace et al. 2019, ApJ 869 L39; Martins et al. 2020, A&A 633 A9]

The high energies involved in the collapse of supermassive stars could trigger the emission of detectable gravitational waves and ultra-long gamma-ray bursts. However, these observational signatures are sensitive to the rotational properties of the star, which depend on the accretion process. [Haemmerlé et al. 2018, ApJ 853 L3; Haemmerlé & Meynet. 2019, A&A 623 L7; Haemmerlé 2021, A&A 650 A204; Haemmerlé 2023, MGM 16 2865]