AB7 in the Small Magellanic Cloud is a binary system composed of two stars, one near the end of its life and the other middle-aged, with immensely powerful winds. Simulations show that such winds could help a star survive if its companion exploded in a supernova that produces a black hole. Credit: ESO
The European Space Agency’s Gaia emission revealed two unexpected black holes orbiting stars like our Sun. One small problem: We’re not exactly sure how black holes like this should form. But a team of researchers may have an answer.
Astronomers cannot observe black holes directly. This is because by definition they do not emit any radiation, and since space itself is equally black, we cannot take a photo. So normally astronomers can only gather evidence of the existence of black holes indirectly. The most common method is when black holes orbit a massive star and the black hole’s gravity is able to distort some of the companion star’s atmosphere. When gas pours into a black hole’s event horizon, it heats up and emits high-energy radiation.
Recently, however, the Gaia mission provided the first glimpses of an entirely new class of black hole systems. The two newly discovered black holes have not been found directly. Instead, astronomers scanned the treasure trove of more than 3 billion stellar measurements captured so far with the spacecraft. Within this data they found two systems in which the stars obviously orbited some other object, yet there was no visible companion.
Based on the orbital characteristics of those stars, the hidden companions must have a mass eight to nine times that of the Sun. The only type of astrophysical object that fits this description is a black hole.
But these two black holes, nicknamed BH1 and BH2, pose a fun astrophysical challenge. Their stellar companions are about the same mass as the Sun and have about the same amount of heavy elements. But they find themselves orbiting these black holes in very long, elongated orbits.
Simply put, these types of systems shouldn’t exist. To create a black hole you need to start with a massive star. But at the end of their lives, massive stars are incredibly unstable, emitting violent winds and expanding into red giants. When this happens, a smaller, Sun-like companion usually loses out. Sometimes the more massive star swells so much that its outer atmosphere envelops the smaller star, causing it to merge with the black hole. In other cases, the ejection of material causes the smaller star to simply be ejected from the system altogether. And in some cases so much material pours onto the smaller star that it transforms it into a more massive one.
In any case, astronomers don’t expect to see small stars orbiting black holes. But that’s exactly what the data reveals, so astronomers are forced to find a solution. Recently, a team of astronomers did just that in a paper submitted to the journal Astronomy and astrophysics.
A path to survival
To examine the mysterious case of BH1 and BH2, they turned to computer simulations. The team wanted to investigate whether there were scenarios in which a massive star could end its life and go supernova while a smaller companion remains intact.
The clue lies in metallicity, which is a term for the amount of heavy elements inside a star. For both BH1 and BH2, the smaller stars have relatively high Metallicities. So it stands to reason that their long-dead comrades also had high levels of heavy elements.
Using sophisticated computer simulation code that tracks the evolution of stars, including their interiors and the material that escapes their reach, astronomers have found a certain special class of stars that could explain this type of system. If stars were massive enough, at least 80 solar masses, then their high metallicity would mean they would be able to transfer a lot of internal energy in incredibly strong winds. In fact, these winds may be so strong that when the star swells into a red giant, it doesn’t get as big as it otherwise might. All of this means that there is an efficient path for supergiant stars to become moderately sized black holes like BH1 and BH2.
Astronomers next looked specifically at whether it was possible for a small companion to survive the onslaught of ferocious winds without becoming destabilized. If this were possible, then there might be a path for the larger companion to turn into a black hole, but it would allow for the existence of a sun-like companion.
Astronomers not only found that this was possible, but they also discovered that it may be a relatively common finding. In other words, we shouldn’t be surprised that systems like BH1 and BH2 exist. The astronomers behind the study suspect that there may be hundreds of such systems hidden in the existing Gaia data that we have yet to discover.
To test this scenario, the team suggests continuing the hunt for more of these strange binary systems, paying particular attention to the lengthening of their orbits.
These new observations and the proposed solution show us how rich and complex stellar evolution is. With each passing year, astronomers learn more about how black holes can form and exist, teaching us about star formation, the evolution of solar systems, and the future of star formation itself.
