A never before seen way to annihilate stars
(Nanowerk News) Most stars in the universe die in predictable ways, depending on their mass. Relatively low-mass stars like our Sun peel off their outer layers in old age and eventually fade to become white dwarfs. More massive stars burn brighter and die faster in violent supernova explosions, creating ultra-dense objects like neutron stars and black holes. If two such stellar remnants formed a binary system, they too could eventually collide. However, new research points to a fourth option that has long been hypothesized, but had not been seen before.
While searching for the origin of long duration gamma ray bursts (GRBs), astronomers using the Gemini South Telescope in Chile, part of the Gemini International Observatory operated by NOIRLab NSF, and other telescopes, have found evidence of a demolition derby. -such as stellar collisions or stellar remnants in chaotic, dense regions near supermassive black holes of ancient galaxies.
“These new results show that stars can meet their end in some of the densest regions of the universe where they can be driven to collide,” said Andrew Levan, an astronomer at Radboud University in the Netherlands and lead author of the paper that appears in the journal Natural Astronomy (“Long-duration gamma-ray bursts originating from ancient galactic nuclei”). “It’s exciting to understand how stars die and to answer other questions, such as what unexpected sources can create the gravitational waves we can detect on Earth.”
The ancient galaxy is long past its star formation peak and will have few, if any, giant stars left, the main source of long GRBs. Their cores, however, are full of stars and very dense stellar remnants, such as white dwarfs, neutron stars, and black holes. Astronomers have long suspected that in the honeycomb of turbulent activity surrounding a supermassive black hole, it was only a matter of time until two stellar objects collided to produce a GRB. However, the evidence for that type of merger is elusive.
The first hint that such an event had occurred was seen on October 19, 2019 when NASA’s Neil Gehrels Swift Observatory detected a bright flash of gamma rays that lasted for just over a minute. Any GRB that lasts longer than two seconds is considered “long”. Such bursts usually result from the supernova death of a star at least 10 times the mass of our Sun — but not always.
The researchers then used Gemini South to make long-term observations of the faint glow of the GRB to learn more about its origins. The observations allowed astronomers to locate the GRB to a region less than 100 light years from the ancient galactic core, which places it very close to the galactic supermassive black hole. The researchers also found no evidence of a corresponding supernova, which would have left its mark on the light studied by Gemini South.
“Our follow-up observations told us that rather than being a massive star collapsing, the explosion was most likely caused by the merger of two solid objects,” Levan said. “By pinpointing its location to previously identified ancient galactic centers, we have the first tantalizing evidence of new paths for stars to meet their end.”
In normal galactic environments, the production of long GRBs from colliding stellar remnants such as neutron stars and black holes is considered increasingly rare. However, the ancient galactic core was anything but normal and there could have been a million or more stars crammed into a region just a few light years away. Such extreme population densities might be large enough that the occasional stellar collision could occur, especially under the titanic gravitational influence of a supermassive black hole, which would disrupt the motion of the stars and send them in random directions. Eventually, these wayward stars will intersect and merge, triggering a giant explosion that can be observed from great cosmic distances.
It is possible that such events occur regularly in equally dense regions throughout the Universe, but went unnoticed until recently. A possible reason for their obscurity is that the galactic center is full of dust and gas, which could obscure the initial GRB flash and the resulting flare. This particular GRB, identified as GRB 191019A, may be a rare exception, enabling astronomers to detect the explosion and study its after effects.
Researchers want to find more of these events. Their hope is to match GRB detections with suitable gravitational wave detections, which will reveal more about their true nature and confirm their origins, even in the dismal environments. The Vera C. Rubin Observatory, when it comes online in 2025, will be invaluable in this kind of research.
“Studying gamma-ray bursts like this is a great example of how the field is really being advanced by the many facilities working together, from the detection of GRBs, to discovering the glow and distance with telescopes like Gemini, to dissecting the details of events. with observations across the electromagnetic spectrum,” Levan said.
“These observations add to Gemini’s rich heritage for furthering our understanding of stellar evolution,” said Martin Still, NSF program director for the International Gemini Observatory. “Time-sensitive observations are a testament to Gemini’s agile operations and sensitivity to distant dynamic events across the Universe.”
