Tuesday, November 24, 2015

Quintenary Star System: Anything But Ordinary

We’ve all heard about binary stars (one star revolving around another). The most popular binary stars are Mizar and its fainter companion star Alcor. To find them, look for the Big Dipper and find the handle; you’ll spot Mizar first as the middle star of the Big Dipper’s handle. Look closely and you’ll see Alcor right next to Mizar.

Harder to find are the trinary star systems, or triple star systems. It’s interesting to me because usually two of the stars form a close binary system while the third star orbits this pair at a distance in a much larger orbit. One such system is the Alpha Centauri star system with Proxima Centauri at a distance of 4.22 light years. It is the closest star to Earth beyond our own Sun.   

The Kepler Space Telescope observed KIC 2856960 for four straight years before astronomers seemed to have enough data to make a determination. But when three astronomers, Marsh, Armstrong, and Carter in the UK started digging into the numbers, they discovered the system was anything but ordinary. First they observed two stars in close orbit of one another: a binary system. The smaller star seemed to orbit the larger one every 6 hours. However, there appeared to be a third star that entered into the equation every 204 days. This was throwing off their results. As they diligently worked out their dilemma on paper, other astronomers became involved. It’s the fact that they could not find a precise set of masses and radii for the component stars. We can’t explain why this star system is producing an ‘impossible’ light curve.

As the team studied the data from KIC 2856960 they saw a small dip in brightness about 4 times a day and a larger dip every 204 days. Okay, not a big deal; it’s a triple star system, so let’s move on to other data. But they were drawn back to their results to look at it with even more detail. After all, this was tricky, and there were all sorts of things that threw off results, such as starspots and other stellar activity. The more they looked at the data, the more confusing things got. At first glance, it looked like a triple star system, but the stars’ orbits didn’t seem to fit. They kind of fit, but there were fluctuations in the data. So the team tried another scenario and found that a four-star system worked—a quintenary system—two sets of binary stars!

This is anything but ordinary, and further data needs to be collected, but it’s clear that this is not a simple, boring triple-star system. We just don’t understand it yet. Perhaps the Transiting Exoplanet Survey Satellite, or TESS, will be able to shed some light on this. TESS is scheduled to launch in 2017. Until then KIC 2856960 is likely to remain a mystery. Is this a quintenary star system? Is there a planet hidden in this system? Don’t you love a good mystery?

My sources: AtlasoftheUniverse.com, space.com, phys.org, pbs.org, atrobites.org, astronomy.com, and American Association of Variable Star Observers (AAVSO)