Black-hole binary tests supernova theory

Combining observations of a newly discovered binary star system with sophisticated models of stellar collapse provides important insights into the formation of stellar-mass black holes. An international team, including researchers from the ORIGINS cluster, concludes that stellar black holes can form even without a bright supernova explosion. The energy of the collapse is mainly emitted by light neutrinos with only a small asymmetry, resulting in a small recoil for the newly born black hole.

A small black circle over a large bright circle. An artist’s impression of a binary system with a black hole and a star.

This artist’s impression shows what the binary system VFTS 243 located in the Tarantula Nebula in the Large Magellanic Cloud might look like if we were observing it up close. The sizes of the two binary components are not to scale: in reality, the blue star is about 200 000 times larger than the black hole. Note that the 'lensing' effect around the black hole is shown for illustration purposes only, to make this dark object more noticeable in the image. Image: ESO/L. Calçada

Snapshot of a three-dimensional simulation of a supernova based on a stellar model with mass 11.2 times heavier than the Sun. Convective overturns are visible as neutrino-heated matter expands in mushroom-like buoyant plumes. Image: Tamborra et al. (2014)

Stars that are several times more massive than the Sun end their lives in powerful and luminous explosions known as supernovae. The collapse of the dense metal core of the massive star releases a large amount of energy, mostly in neutrinos, while the outer layers of the star are expelled into outer space. This material can amount to many times the mass of the Sun and is ejected with velocities of hundreds to thousands of kilometres per second, leading to large-scale asymmetries of the ejected matter that we also observe in the remnants of the explosions.

These asymmetries and mass ejecta directly affect the very dense remnant at the core, the newly formed neutron star, which experiences a recoil – a natal kick – that can abruptly change its velocity. There is plenty of evidence of these natal kicks for neutron stars, as we observe them moving at large speeds throughout the Milky Way. However, for the most massive compact objects known, black holes, these natal kicks are not well understood. Such stellar black holes form in the collapse of massive stars, in particular when the explosion does not succeed and the in-falling matter collapses onto itself. 

The recent discovery of “disappearing” stars suggests that a large fraction of collapsing massive stars form black holes without any explosion, which unlike the bright supernovae we cannot observe. However, it is unclear how much mass these stars lose during black hole formation, or how large their natal kicks are. If the massive star directly collapses into a black hole, no baryonic matter is ejected, and energy is predominantly lost via neutrinos.

The team investigated the complete collapse scenario for the black hole binary VFTS 243, in which a star ten times more massive than the Sun ended its life cycle by imploding. With state-of-the-art models of stellar collapse developed at MPA in the group led by ORIGINS scientist Prof. H.-Thomas Janka, they calculated the effects on the orbit of a binary star system during the black hole formation. In the complete collapse scenario, the enormous gravitational binding energy released during the formation of the black hole is carried away exclusively by the weakly interacting, neutral and light neutrinos. The black hole observed in the binary star system VFTS 243 allowed the conclusion, for the first time, that neutrinos are emitted almost equally in all directions when the massive progenitor collapses to form the black hole.

Press release MPA

A. Vigna-Gómez, R. Willcox, I. Tamborra, et al.  “Constraints on neutrino natal kicks from black-hole binary VFTS 243”, Physical Review Letters, 2024

Prof. Dr. H.-Thomas Janka
Max-Planck-Institut für Astrophysik / Excellence Cluster ORIGINS