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Impossible, record breaking orbits for red dwarfs
Posted: 14 July 2012

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Red Dwarfs A binary red dwarf system with an ultra-short orbital periods will eventually merge into a single star. Image: J. Pinfield, for the RoPACS network.

The United Kingdom Infrared Telescope has unveiled a puzzling set of binary stars that whiz around each other in time-scales that were thought to be impossible. These unexpected results could mean that its back to the drawing board for understanding red dwarf formation.

Around half of the stars in the Milky Way are in binary systems, where two stars orbit each other. Some of these main sequence binary stars orbit each other in a matter of hours, but it seemed that no binaries existed with orbital periods less than around five hours. This is what made the discovery of several binary star systems with periods less than four hours surprising.

The four binary star systems contain M-dwarf stars, or red dwarfs, which are stars that are up to ten times smaller than our Sun. They are also less luminous, and their dim red glow is barely discernible in telescopes that are optimised for visible light. These stars emit most of their light at infrared wavelengths, which is why the state-of-the-art Wide Field Camera (WFCAM) on the 3.8 metre United Kingdom Infrared Telescope (UKIRT) was used to survey them.

Determining the distribution of binary orbital periods is vital for studying the formation and evolution of these stars. The five hour cut off period for M dwarf binaries was thought to be due to a very slow time-scale of orbital decay. M dwarfs are very long lived, and this theory assumed that the binaries simply haven't reached this stage in their evolution yet.

Red dwarfs are typically very active stars, and produce strong magnetised winds. These stellar winds reduce the angular momentum of the binaries, which causes the stars to move closer to each other as their orbital period decreases. However, it now looks likely that either these timescales have been overestimated and that the red dwarfs may be more active than previously assumed. If the stellar activity is greater, than the "magnetic braking" will cause the stars to spiral into closer orbits sooner than expected. Another possibility is that M-dwarf binaries have a different formation method compared to larger main sequence stars.

These four unusual binary systems will experience immense tidal forces as they creep closer to their partners. "This can actually affect their shape making them slightly oblate in extreme cases," explains David Pinfield, a principal investigator of the WFCAM Transit Survey. "Such binary systems will merge into a single star over a time-scale of around a few thousand years."

The main goal of the WFCAM Transit Survey (WTS) is to find planets transiting M-dwarf stars, and the survey monitors the brightness of hundreds of thousands of stars, including 10,000 M-dwarfs. Unfortunately the UKIRT will cease operations in 2013 due to a lack of funding, which will see the end of WTS before completion as well as other innovative science being performed with the telescope.

"WTS is a time-domain survey and our goal has always been to obtain 1000 measurements of every targeted star - its this plethora of measurements that allows us to identify the most interesting objects," Pinfield tells Astronomy Now. "We only expect to achieve this coverage for half the surveyed stars, with a significant short-fall for the remaining ones."

The paper will be published in Monthly Notices of the Royal Astronomical Society, and the preprint of the paper can be found here:

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