An artist's conception shows two M4 red dwarfs orbiting each other… (J. Pinfield/RoPACS network )
Astronomers using the United Kingdom Infrared Telescope in Hawaii have discovered four pairs of binary stars doing something researchers had previously believed to be impossible: orbiting each other with periods of less than four hours. One whirling dervish pair whip around each other in only 2.5 hours, a remarkably short period for such massive objects. Astronomers had previously believed that any binaries orbiting in such close proximity would coalesce into a single star. That may eventually prove true, but it appears to be happening more slowly than expected -- at least in some cases.
About half the stars in the galaxy are binaries, orbiting each other with periods ranging from hours to years. Most likely, the stars were formed close together and have been orbiting each other ever since. Previous studies have monitored such stars in visible light and have found that the shortest orbital period is never less than five hours.
The U.K. telescope, whose 3.8-meter (150-inch) mirror sits atop Mauna Kea, is the largest exclusively infrared telescope in the Northern Hemisphere. By searching in the infrared, it can detect red dwarfs, which can be as small as a tenth the size of the sun and have only one-thousandth its brightness. Red dwarfs are the most common form of star in the Milky Way, but they cannot be seen in normal telescope images because they are too dim.
A team headed by astronomer Bas Nefs of the Leiden Observatory in the Netherlands reported in the journal Monthly Notices of the Royal Astronomical Society that they observed four pairs of binary stars with periods shorter than four hours, including one with a period of only 2.5 hours. "It means that we have to rethink how these close-in binaries form and evolve," Nefs said.
Because these stars shrink over the course of their lifetime, their orbits must also have shrunk. Otherwise, the stars would have been in contact early in their lives and would have merged. But it is not clear how the orbits could have shrunk so much, Nefs said. Possibly, the cool stars in binary systems are much more active and violent than previously suspected, he added. Magnetic field lines radiating out from the stars could get twisted and deformed as they spiral toward each other, generating extra activity through stellar winds, explosive flaring and star spots. Such magnetic activity could slow the planets, causing their orbits to shrink.