05 October 2013

Researchers Discover Third Star In Fomalhaut System

Image Credit: ALMA (ESO/NAOJ/NRAO). Visible light image: the NASA/ESA Hubble Space Telescope 

The search for exoplanets – worlds outside of our own solar system – has grown exponentially in the last decade or two. And while news about the hunt for planets that could potentially support life dominates the headlines, it is equally important to understand the star systems that host these distant space rocks.

One system that has garnered significant attention is the nearby star system known as Fomalhaut, partially because it contains a peculiar exoplanet, but also because it was found to be a double star system, meaning that it has two stars that are gravitationally bound together. However, new results indicate that rather than two stars, the system is actually a triple star system, with a third star orbiting farther out from the other two.

“I noticed this third star a couple of years ago when I was plotting the motions of stars in the vicinity of Fomalhaut for another study,” reports Eric Mamajek, associate professor of physics and astronomy at the University of Rochester. “However I needed to collect more data and gather a team of co-authors with different observations to test whether the star’s properties are consistent with being a third member of the Fomalhaut system.”

Mamajek was able to get the additional confirmation he needed to move forward with his hypothesis. While attending a conference in Chile, he found himself in a conversation with Todd Henry from Georgia State University and director of the Research Consortium On Nearby Stars (RECONS) team.

“Eric was playing detective on this third star and I just happened to be sitting there with an observing list that contained the unpublished parallax [a method for reporting the distances to astronomical objects],” Henry said. “A student at the time, Jennifer Bartlett at the University of Virginia, was working with us on a sample of potentially nearby stars for her PhD thesis, and LP876-10 was on it. Eric and I got to talking, and here we are with a cool discovery.”

Using careful measurements of the third star, the astronomers were able to find the distance to the object with a relatively high precision. From this they could show that the stars in the system were close enough to gravitationally interact. “Fomalhaut A is such a massive star, about twice the mass of our Sun, that it can exert sufficient gravitational pull to keep this tiny star bound to it – despite the star being 158,000 times farther away from Fomalhaut than the Earth is from the Sun,” Mamajek said.

Part of the reason that this was not discovered much sooner was that “Fomalhaut C looks quite far apart from the big, bright star that is Fomalhaut A when you look up at the sky from Earth,” added Mamajek. In fact, plotted onto the night sky, the Fomalhaut C appears to be about 11 moon diameters away from the main star. While this seems paradoxical that objects appearing so far apart from each other could be part of the same system, the illusion arises because the system is so close to Earth, at roughly 25 light-years.

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