Surprisingly massive black hole with stellar mass discovered in nearby binary star system

Using data from ESA’s Gaia mission, astronomers discovered a nearby binary system of massive stars orbiting a dormant black hole of stellar origin after 11.6 years. The black hole’s estimated mass (33 solar masses) is significantly larger than the masses of known stellar-mass black holes in the Milky Way, and within the mass range of extragalactic black holes detected by gravitational waves. The binary star system in question is called Gaia BH3 and is located 1,926 light years from Earth in the constellation Aquila. Also known as Gaia DR3 4318465066420528000, LS II +14 13, and 2MASS J19391872+1455542, it consists of an old, very metal-poor giant star and a dormant stellar-mass black hole. Gaia BH3 is his third dormant black hole discovered by ESA’s Gaia interstellar mapping satellite. “This is the kind of discovery that only happens once in a research career,” he said. Pasquale Panuzzo, astronomer at CNRS and Paris Observatory. “So far, black holes this large have only been discovered in distant galaxies by the LIGO-Virgo-KAGRA collaboration, thanks to observations of gravitational waves.” The average mass of the known stellar-origin black holes in our galaxy is about 10 times the mass of the Sun. Astronomers face the urgent task of explaining the origin of black holes as large as Gaia BH3. Our current understanding of how massive stars form and die does not immediately explain how this type of black hole formed. Most theories predict that massive stars eject significant amounts of material through strong winds as they age. Eventually, it is partially ejected into space when it explodes as a supernova. Depending on its mass, the rest of the core continues to shrink, becoming either a neutron star or a black hole. It is very difficult to explain a core large enough to end up in a 30 solar mass black hole. But the clue to this mystery may lie very close to Gaia BH3. This star, which orbits Gaia BH3 at about 16 times the distance between the Sun and Earth, is quite rare, having formed during the first two billion years after the Big Bang, when our galaxy began to form. It is an ancient giant star. It belongs to the family of galactic star halos, which move in the opposite direction to the stars in the galactic disk. Its orbit suggests the star may have been part of a small galaxy or globular cluster that was swallowed up by the Milky Way more than 8 billion years ago. This companion star contains almost no elements heavier than hydrogen or helium, suggesting that the massive star that became Gaia BH3 may also have been extremely poor in heavy elements. For the first time, the theory that the massive black holes observed in gravitational wave experiments were formed by the collapse of massive protostars lacking heavy elements has been confirmed. These early stars may have evolved differently from the massive stars we see in our galaxy today. The composition of the companion star also provides information about the formation mechanism of this surprising binary system. “What struck me was that the chemical composition of the companion star is similar to that found in ancient, metal-poor stars in galaxies,” he said. Elisabetta Cuffo, astronomer at CNRS and Paris Observatory. “There is no evidence that this star was contaminated by material ejected from the supernova explosion of the massive star that formed BH3.” “This could indicate that the black hole acquired a mate after its birth, capturing it from another star system.” The team’s research is published in the journal Astronomy & Astrophysics.

source: https://doi.org/10.1051/0004-6361/202449763