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.

R CrB 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
30=AD120 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”.

NOTES

1. A preprint of the paper reported here may be found at:
http://www.arm.ac.uk/~csj/papers/mb139.pdf

2. An animated illustration of the merger of two white dwarfs may
be found on the web page:
http://www.arm.ac.uk/~csj/movies/merger.html