Just over two years ago, in July 2022, an international team of astronomers discovered, outside our galaxy, a strange black hole. Like many others, it was part of a binary system, that is, it was gravitationally linked to another star. But unlike all other black holes in the same situation, this system, classified as VFTS 243 and located in the Large Magellanic Cloud, a small satellite galaxy of our Milky Way, was not ‘feeding’ on its companion. , but remained totally inactive. The very massive star is 25 times the mass of the Sun and its ‘partner’, the black hole, is approximately 10 solar masses. The unusual find was then published in ‘Nature‘.
Now, another team of scientists, this time from the Max Planck Institute for Astrophysics and the Niels Bohr Institute at the University of Copenhagen, has thoroughly studied the peculiar binary system and has found, for the first time, evidence that black holes can form. ‘directly’ after the collapse of a very massive star, that is, without going through the dramatic and usual explosion as a supernova.
A different binary system
For years, astronomers have known about binary star systems in the Milky Way in which a star is paired with a black hole. «The discovery of the binary black hole VFTS 243 in our neighbor Large Magellanic Cloud –says Alejandro Vigna-Gómez, first signatory of an article published in ‘Physical Review Letters‘- was extraordinary, and the system itself is remarkable.’
Experience tells us that stars that are several times more massive than the Sun often end their lives as supernovae, violent explosions that for a time outshine all the stars in a galaxy. During these events, the dense metallic core of the star collapses, sinking in on itself, releasing immense energy, mainly in the form of neutrinos. The star’s outer layers are then violently ejected into space at speeds of hundreds to thousands of kilometers per second. This ejected material, which can equal several times the mass of the Sun, creates large-scale asymmetries in the remnants of the explosion, observable even long after the supernova occurs.
These asymmetries and enormous mass ejections directly affect the extraordinarily dense remnant of the core, a newly formed neutron star, which experiences a violent recoil – a ‘natal kick’ – that abruptly alters its speed. There is abundant evidence of these impulses from neutron stars, as we observe them moving at high speeds throughout the Milky Way. However, in the case of black holes born from a direct collapse of the star, without any explosion, these asymmetries, and the associated ‘natal kicks’, are minimal.
Disappeared stars
The recent discovery of stars that appear to suddenly ‘disappear’ without leaving a trace suggests that the number of massive stars that collapse in this way, forming black holes without exploding, must be enormous although, unlike bright supernovae, they do not. we can see them. However, it is not clear how much mass these stars lose during the formation of black holes, nor how big their ‘natal kicks’ are. Logically, if the massive star collapses directly into a black hole, no matter is ejected into space and energy is lost predominantly through neutrinos. “The VFTS 243 system has allowed us to test this scenario,” says Vigna-Gómez.
There, in fact, a star ten times more massive than the Sun ended its days as a result of an implosion. Using the most modern models of stellar collapse developed at the Max Planck Institute for Astronomy, the researchers calculated the effects on the orbit of a binary star system during the formation of the black hole. And in the total collapse scenario, the enormous gravitational energy released during black hole formation is carried exclusively by the ghostly, extremely light particles known as neutrinos.
“Probing the physical processes taking place in the deep interior of collapsing stars is extremely difficult and is only possible under special circumstances,” says H.-Thomas Janka, co-author of the paper. The black hole observed in the binary system VFTS 243, adds co-author Daniel Kresse, “is a very special case.” “It allowed us to conclude, for the first time, that neutrinos are emitted almost equally in all directions when the massive progenitor collapses to form the black hole.”
Therefore, concludes Vigna Gómez, «our study is an excellent example of the synergy between theory and observation. “Combining advanced numerical models of stellar collapse with the principles of supernovae in binary star systems allowed us to obtain crucial information about the complete collapse scenario, in particular by demonstrating that massive black holes can form without the need for an explosion.”
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