Mystery of R Coronae Borealis and other helium stars solved

Astronomers Dr Simon Jeffery of the Armagh Observatory and Dr Hideyuki Saio of Tohoku University, Japan, have finally solved a long-standing mystery concerning the creation of two particular kinds of rare stars. They have found that a class of variable stars named after their prototype R Coronae Borealis (RCrB), and a related group called `extreme helium stars` are the products of mergers between pairs of white dwarf stars. What kind of star results from the merger depends on the composition of the white dwarfs. The research is to be published in the Monthly Notices of the Royal Astronomical Society.

RCrB stars and their hotter cousins, the extreme helium stars, are highly unusual. While most ordinary stars are typically three-quarters hydrogen (by weight), these oddities have hardly any hydrogen on their surfaces. Instead, they are made primarily of helium, with some carbon, traces of hydrogen and other peculiarities. For some time, astronomers have suspected that they are the mixed-up remains from inside old stars, where nuclear fusion has created helium, carbon and other chemical elements. The question has been, how did it happen?

The problem has haunted Simon Jeffery for much of his career. He began studying extreme helium stars about 20 years ago, and his collaboration with Hideyuki Saio started in 1985. A breakthrough came when Jeffery realised that the helium stars are giving out more energy than they produce inside them by nuclear processes. That meant they must be shrinking. Observations he made of four helium stars with the orbiting International Ultraviolet Explorer (IUE) observatory demonstrated that they were getting hotter by 30120 degrees per year. And observations of some pulsating helium stars showed that they are 90% the mass of the Sun.

Saio, an expert on computer modelling, developed the simulations of stellar mergers needed to convince other astronomers that two white dwarfs coming together could explain the observations. It was a difficult job. Conventional thinking said that if you added hydrogen from one white dwarf to another, it would either just be blown away or there would be a supernova explosion. But what would happen if you added helium?

White dwarfs are the cores left over when old, evolved stars blow off their outer layers. They are by no means all the same and their compositions cover a bewildering range. A simulated merger between two helium white dwarfs produced a star matching very closely the properties of a nitrogen-rich helium star called V652 Herculis. A merger between a carbon-oxygen white dwarf and a helium white dwarf matched the shrinking helium stars Jeffery had observed with IUE and explained very well the properties of RCrB stars and extreme helium stars.

“There are still some unanswered questions, though” says Jeffery. “The actual merger, when one white dwarf is ripped apart by its companion, is
likely to be extremely violent, taking a matter of a few minutes. We don`t yet know how the material will be spread out – into a big disk around the star perhaps – or what happens as the new helium star expands by a factor of 10,000″.