Team spots 'impossible' binary stars

Posted by Kate Taylor

A team of astronomers has discovered four pairs of stars that they thought couldn't exist, with the stars in each pair orbiting one another in less than four hours.

About half the stars in our Milky Way galaxy are are such binary systems, but none has ever been found with an orbital period of less than five years; and it was thought that below this level, they'd quickly merge into one single, bigger star.

But, using the United Kingdom Infrared Telescope (UKIRT) on Hawaii, the team investigated binaries of red dwarfs for the first time.

"To our complete surprise, we found several red dwarf binaries with orbital periods significantly shorter than the 5 hour cut-off found for Sun-like stars, something previously thought to be impossible," says Bas Nefs from Leiden Observatory in the Netherlands.

"It means that we have to rethink how these close-in binaries form and evolve."

Since stars shrink in size early in their lifetime, the fact that these very tight binaries exist means that their orbits must also have shrunk since their birth - otherwise, they'd have been in contact early on and would have merged.

However, what's got the astronomers scratching their heads is the question of how these orbits could have shrunk by so much.

One possibility is that cool stars in binary systems are much more active and violent than previously thought.

The magnetic field lines radiating out from the cool star companions may get twisted and deformed as they spiral in towards each other, generating extra activity through stellar wind, explosive flaring and star spots.

Powerful magnetic activity could apply the brakes to these spinning stars, slowing them down so that they move closer together.

"Without UKIRT’s superb sensitivity, it wouldn’t have been possible to find these extraordinary pairs of red dwarfs," says team member David Pinfield.

"The active nature of these stars and their apparently powerful magnetic fields has profound implications for the environments around red dwarfs throughout our galaxy."