Two burned-out stellar cores collide and merge into one. That's how R Coronae Borealis stars are born — rare luminaries where oxygen-18 is hundreds of times more abundant than in the Sun. But exactly how this happens has remained a puzzle.
The researchers took a computer model of a merger between two white dwarfs — one made of helium, the other of carbon and oxygen. They tracked how the amount of oxygen-18 changes in different layers of the newborn star. It turned out that in the outer layers of the burning shell, oxygen-18 is produced not from nitrogen-14, as previously thought, but from carbon-14. This pathway operates for a longer time, so oxygen-18 has time to accumulate in large quantities.
This picture explains why, in R Coronae Borealis stars, the ratio of oxygen-16 to oxygen-18 can be close to one — hundreds of times lower than in the Solar System. It all comes down to which reactions occur in different layers after the merger.
In the Solar System, there are about 500 atoms of oxygen-16 for every atom of oxygen-18, but in R Coronae Borealis stars, this ratio can be nearly one to one.