Extreme conditions prevail around the black hole at the center of the Milky Way – enormous tidal forces, turbulence and strong radiation. According to current assumptions, no new stars can form there. But now astronomers have discovered a protostar, complete with circumstellar disk and dust cocoon, in the middle of this supposedly “sterile” environment. The star X3a has a mass of around 15 solar masses and is a few 10,000 years old, as the spectral observations suggest. How this protostar could have formed less than a light-year away from the black hole Sagittarius A* is so far unclear. The astronomers suspect that it may have gotten so close to the black hole only after the collapse of its formation cloud.
The environment surrounding the supermassive black hole Sagittarius A* at the center of our galaxy is anything but calm: the black hole’s massive mass generates enormous tidal forces that threaten to tear apart nearby matter. There is also high-energy X-ray and UV radiation, which heat up the surrounding gas and dust. These conditions usually prevent a star from forming. Because this requires dense accumulations of cold molecular gases. According to current assumptions, there should only be older stars in the vicinity of Sagittarius A*, originally formed somewhere else, which reached the center of the galaxy billions of years ago as a result of turbulence. But contrary to this assumption, astronomers have repeatedly found evidence of young stars near the central black hole. The unexpected appearance of very young stars in close proximity to the supermassive black hole has been dubbed the “paradox of youth.”
Young star hidden in dust cocoon
In order to get to the bottom of this paradox, a research team led by Florian Peißker from the University of Cologne examined several conspicuous objects in the center of the Milky Way with several telescopes and high-resolution spectrometers. One is X3, a cluster of three hot clumps of unclear composition just a third of a light-year from the black hole. One of these clumps turned out to be a surprising find, because the cocoon of hot dust and gas hid a young, immature star. “It is a young stellar object with a surrounding disk embedded in a bow-wave cocoon of dust,” the astronomers report. The young star lying in the center of this system is about 15 times as heavy as our sun. “With its large mass, X3a is a giant among the stars, and these giants are evolving into a mature star very quickly. We were lucky to spot the massive star in the middle of the comet-shaped circumstellar envelope,” says co-author Michal Zajaček from Masaryk University in Brno.
Spectral analyzes revealed that X3a is a so-called Herbig-Ae/Be star that is only a few 10,000 years old. These protostars are not yet dense and heavy enough to ignite hydrogen fusion and derive their energy from the infall of large amounts of matter. Although X3a is less than a light-year from the black hole, this young star has managed to largely capture its circumstellar disk and dust cocoon. The astronomers attribute this to the young star’s large mass: “X3a’s large mass protects the inner region of the system, although we have located Sagittarius A*’s gravitational footprint at just 0.32 light-years away,” they explain.
How did X3a come about?
However, X3a should not really exist that close to the supermassive black hole. Because even if the young star now has enough mass not to be torn apart again immediately, the question remains as to how it could have formed so close to the black hole. So far, astronomers can only speculate. One possible explanation is that a cold molecular cloud of gas tumbled into the galactic center, providing raw material for star formation. Another would be that the star formed in a cloud of dust orbiting the gigantic black hole at a slightly greater distance and only then sank to its current orbit. “At a distance of just a few light years from the black hole, there is a region that meets the conditions for star formation,” explains Peißker. “This ring of gas and dust is sufficiently cold and shielded from damaging radiation.”
The scientists therefore assume the following scenario: Shielded from the influence of the black hole’s gravity and its intense radiation, a sufficiently dense gas cloud could have formed in this zone of gas and dust. This cloud had a mass of about a hundred suns and collapsed under its own gravity into one or more protostars. “This so-called fall time roughly corresponds to the age of X3a. Therefore, we assume that the process was the birth of X3a,” says Peißker. Only then did X3a fall to its current position. Since similar rings of dust and gas can also be found in other galaxies, this mechanism could also function there and contribute to star formation around the central black holes. Planned future observations with NASA’s James Webb Space Telescope or the European Southern Observatory’s (ESO) Extremely Large Telescope in Chile could show whether this model of star formation is possible for our and other galaxies.
Source: Florian Peißker (University of Cologne) et al., The Astrophysical Journal, doi: 10.3847/1538-4357/aca977