Amateur astronomers have helped to discover an exceptional pair of stars. The highly evolved pair resembles a system that will later become a supernova, and is perfectly aligned with Earth for studying. Astronomically speaking, the stars are not even very far away, at 730 light-years.
More than 80 authors are on the paper, called "Total Eclipse of the Heart", published in the Monthly Notices of the Royal Astronomical Society. It describes an eclipsing binary, where one of the stars blocks the view of the other as seen from Earth, and they complete an orbit of each other every 50 minutes.
One of the stars is a white dwarf, about the size of Earth but much more dense. It's companion is about 125 times the volume of our Sun, but has just 1% of the white dwarf's mass. The white dwarf is taking considerable amounts of material from its companion, which will likely result in a Type 1a supernova.
Type 1a supernovae occur when white dwarfs grow heavy enough to explode. They can be used as “standard candles” to measure cosmic distances, and they allow us to map the scale of the universe. However, debates about how they're triggered have intensified recently, with competing theories each building support.
Other categories of supernovae are preceded by enormously large and bright stars, providing opportunities to investigate objects before they explode. But white dwarfs are so faint that only the closest future Type Ia supernovae are visible. At 730 light-years away in the Draco constellation, this combination of a white dwarf and puffed up companion known as Gaia14aae is close enough to be seen in amateur telescopes.
Gaia14aae is also unusual because the atmospheres of both stars are rich in helium, while lacking hydrogen, placing the pair in the very rare AM Canum Venaticorum (AM CVn) category of stars. Moreover, this is the first known AM CVn pair where one star entirely eclipses the other as seen from Earth, allowing us to collect more accurate information on the radii and masses of the stars as light from one passes around the other.
As the name suggests, the binary system was originally discovered by the Gaia space telescope after a sudden flaring to five times its previous brightness. Follow-up observations were done by the Center for Backyard Astrophysics (CBA), a project that allows amateur astronomers to collaborate with professionals to conduct scientifically valuable research.
It was the CBA that identified Gaia14aae's eclipsing binary nature. "It's really cool that the first time that one of these systems was discovered to have one star completely eclipsing the other, that it was amateur astronomers who made the discovery and alerted us," said lead author on the paper Dr Heather Campbell of the University of Cambridge.
When white dwarfs draw material off companion stars but haven't gained enough mass to go supernova, they can experience explosions on parts of their surface causing them to brighten to nova status. This may be what's happening here. Estimates of the two stars' mass suggests the system probably lacks sufficient mass to become a supernova. However, in other ways the two stars closely resemble the sort of system that is thought to eventually become a Type Ia supernova, giving us a great chance to study the behavior of these systems.
"This is an exquisite system: a very rare type of binary system in which the component stars complete orbits faster than the minute hand of a clock, oriented so that one eclipses the other," said Professor Tom Marsh of the University of Warwick. "We will be able to measure their sizes and masses to a higher accuracy than any similar system; it whets the appetite for the many new discoveries I expect from the Gaia satellite."
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