Enlarge /. Artistic representation of the orbits of the objects in the HR 6819 Triple System. There is an inner binary file with a star (orbit in blue) and a newly discovered black hole (orbit in red) and a third star in a wider orbit (also in blue).
ESO / L. Calçada
Astronomers from the European Southern Observatory (ESO) have discovered a black hole that is the closest object that has been found so far and is only 1,000 light-years away – close enough to be seen with the naked eye. It is part of a triple star system called HR 6819, and ESO scientists believe that other members of this system class may do so Likewise are home to black holes that were previously not a high priority for finding black holes. They announced their discovery in a new article published in the journal Astronomy and Astrophysics.
Scientists believe that there are far more black holes in the universe than we have discovered so far – probably hundreds of millions of them given the age of our universe – because we cannot observe them directly; we can only deduce their presence by their effect on the surrounding matter. For example, the gravitational effects of a black hole can affect the orbits of neighboring stars, or infalling matter can form a hot gas accretion disk that quickly orbits the black hole and emits strong X-rays. Or an unfortunate star gets too close to a black hole and is torn apart for its problems, with the infallible remains also accelerating and warming up to emit X-rays into space.
But most black holes are actually quiet and therefore very difficult to see. This latest discovery provides useful clues as to where at least some of the really dark black holes might be hiding. "You will never have enough time for a telescope to search for all the objects so thoroughly," ESO scientist Thomas Rivinius, co-author of the paper, told Ars. "What you need to do is a tiered approach that will help you "Identify candidates, then dilute the list of candidates, and then look closely at the remaining ones. If we know what to look for, we'll be in a better position to find them."
The ESO team had conducted a study on binary star systems and HR 6819 was included in the collection of observation data because it appeared to be one. However, when reviewing their data, astronomers found clear evidence of an unexpected third object in the system: a black hole that had previously eluded discovery.
In a three-star system, two of the stars orbit a binary pair, while the third star orbits the pair at a greater distance. This ensures that the system is stable since, with the same size of the inner and outer orbit, one of the stars may be ejected from the system. In the case of HR 6819, one of the two visible stars orbits an invisible object every 40 days, while the other visible star orbits further away. By examining the orbit of the star in the inner pair, the team was able to determine the presence of the black hole and also calculate its mass. "An invisible object with a mass at least four times the size of the sun can only be a black hole," said Rivinius.
Wide field view. While the black hole is invisible, the two stars in HR 6819 can be viewed from the southern hemisphere on a dark, clear night without binoculars or a telescope.
ESO / Digitalized Sky Survey 2 with Davide De Martin
Diagram showing the position of the HR 6819 triple system in the Telescopium constellation.
ESO, IAU and Sky & Telescope
Southern sky over La Silla, ESO's first observatory, just after sunset.
ESO / José Francisco Salgado
"We used to think that individual stars were the most common," said Rivinius. "At least for the really massive stars are probably the rarest." This is because the larger the mass of a star, the less likely it is to be alone, and Rivinius suggests that even single massive stars could actually be the survivors of multiple star systems that have been "disturbed" or weaker Having companion stars simply cannot be recognized. Trinary systems like HR 6819 are less common, but also not extremely rare. Physicists currently believe that the supernovae that lead to black holes would actually disrupt the structure of multiples. "However, if a significant number of multiples survive the supernovae, the statistics change," said Rivinius.
"If such a system is in close proximity, it is likely to be common in other regions of the galaxy," said Rivinius. His calculation on the back of the envelope suggests that there could be 2,500 such systems. This will not resolve the huge discrepancy between the black holes we discovered and the number of astronomers believed to be out there. "But considering that we didn't know that there could be a triple like this, it is quite a step," he added. The ESO team has already identified a second star system, which may also be a black hole trinary, although more observational data are needed to confirm this.
The discovery that a black hole can be part of a three-star system is also relevant, since astronomers have suggested that such triple systems could be precursors to binary systems with two black holes or a pairing of black holes and neutron stars. When the partners merge in these binary systems, the violent event sends out gravitational waves that can be recognized by the LIGO collaboration.
"The problem with LIGO detections is that it takes a long time for two black holes in a normal, lonely binary to get close. until they finally merge, "said Rivinius." In fact, it takes longer than your current age of the universe, and we really shouldn't see as many mergers as we do, if that were the only mechanism. But the closer they are, the closer It is (much) faster. "
It is known as the Lidov-Kozai mechanism. For example, it occurs when a narrow binary inner pair has a circular orbit, but not in the same plane as the outer orbit. According to Rivinius, this leads to the inner orbit becoming "more eccentric". "In short, this means that the third body can help the two inner ones to at least temporarily approach each other," he said.
However, this will not be the case with the HR 6819 trinary system. "The two stars in HR 6819 are not massive enough to explode as a supernova and form a black hole," said Dietrich Baade of ESO, another co-author. "Therefore, HR 6819 will never harbor two black holes, and it will never be a full equivalent to the forerunners of gravitational wave events. But it is a useful proxy nearby to investigate."
DOI: Astronomy and Astrophysics, 2020. 10.1051 / 0004-6361 / 202038020 (About DOIs).