Space is fairly empty, so the chance of three stars coming within a close enough distance to disrupt their orbits gravitationally is quite small. The more instances of this happening are found, the less likely it is that they have all occurred through chance encounters.
Dr Jayne Birkby
Astronomers have found evidence of binary systems in which twinned stars orbit each other in less than four hours. Appearing in the Monthly Notices of the Royal Astronomical Society
, the research paper details four pairs of stars involved in these close interactions, discovered with the use of the United Kingdom Infrared Telescope (UKIRT) in Hawaii.
Although roughly half of the stars in the Milky Way are part of a binary system in which they orbit another star, observations from the last thirty years supported the assumption that none existed with an orbital period of less than five hours. It was believed that the two stars would inevitably merge if they had formed any closer together than this.
"To our complete surprise, we found several red dwarf binaries with orbital periods significantly shorter than the five hour cut-off found for Sun-like stars, something previously thought to be impossible", commented lead researcher Dr Bas Nefs from Leiden Observatory in the Netherlands. "It means that we have to rethink how these close-in binaries form and evolve."
I spoke to Dr Jayne Birkby, Research Associate at Leiden Observatory and co-author of the paper, to find out more about red dwarf binaries and how they might be formed. She began with an explanation of the reasoning behind astronomers believing this type of interaction impossible.
"It was thought that, because of the distance separating the stars when they were born, it would take longer than the age of the universe for them to get so close together," Dr Birkby told me.
"Low mass stars have a magnetic wind that comes from the star itself. The interaction between the winds of the two stars in a binary system is what causes them to be dragged together. It is thought that the strength of that effect is limited to a certain point and it therefore wouldn’t have been strong enough to bring them together in the last 13.9 billion years."
Red dwarfs are by far the most common type of star in our galaxy, but with a mass up to ten times smaller than the sun and a thousand times less luminosity they did not become observable until the advent of infrared telescopes such as UKIRT. Hundreds of thousands of stars have been monitored by UKIRT’s Wide Field Camera (WFCAM) over the last five years, and the new finding comes as part of the WFCAM’s Transit Survey.
"WFCAM uses infrared, so it has opened up a whole different wavelength to what we would normally observe in the visible spectrum," remarked Dr Birkby. "For these small stars in particular, the optical wavelengths that can be seen with the eye are very faint. The infrared part of the spectrum is where most of the photons are emitted, and so with an infrared telescope you can collect more photons, hence seeing more of these objects. They appear brighter in infrared than at optical wavelengths."
It is not clear how the orbits of the stars might have shrunk to the extent they must have to rule out the possibility of the pairs merging since their birth. The researchers are entertaining a number of different ideas about how the extreme closeness of these stellar binaries might be explained.
"One of the main considerations is that a third star could either be bound gravitationally to the two stars or have been involved in a sort of fly-by event," said Dr Birkby. "This would cause the two stars within the gravitational interaction to be moved closer together, but you have to consider the chances of such an event happening.
"Space is fairly empty, so the chance of three stars coming within a close enough distance to disrupt their orbits gravitationally is quite small. The more instances of this happening are found, the less likely it is that they have all occurred through chance encounters. While interactions with other stars are a possible explanation, it is not yet known whether that’s the only way."
It is also possible that powerful magnetic activity could cause the stars to slow down and move closer together, but more work will be needed to determine what is really going on. I asked Dr Birkby about the next steps for the research she and her colleagues are carrying out.
"For the particular stars we found, the next step is to try to measure their masses and radii directly," she explained. "What we have at the moment is only an indication that they are extremely close; what we want to see is the stars themselves distorting, such that they are almost in contact. The only way we can do that is to look at their light curves and their radial velocity curves. That way we can measure their masses and radii.
"These are the same techniques that are used to measure the mass and radius of exoplanets. By doing so we will be able to confirm that these stars are actually in contact, which then allows theorists to take this information and match their theories to what is observed in nature."