Surprise! Fomalhaut’s Kid Sister Has a Debris Disk Too

by Shannon Hall on January 21, 2014

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Image Credit: Amanda Smith

An artist’s conception of the Fomalhaut system. Image Credit: Amanda Smith

The bright star Fomalhaut hosts a spectacular debris disk: a dusty circling plane of small objects where planets form. At a mere 25 light-years away, we’ve been able to pinpoint detailed features: from the warm disk close by to the further disk that is comparable to the Solar System’s Kuiper belt.

But Fomalhaut never ceases to surprise us. At first we discovered a planet, Fomalhaut b, which orbits in the clearing between the two disks. Then we discovered that Fomalhaut was not a single star or a double star, but a triplet.  The breaking news today, however, is that we have discovered a mini debris disk around the third star.

Fomalhaut is massive, weighing in at 1.9 times the mass of the Sun. And at such a close distance it’s one of the brightest stars in the southern sky. But its two companions are much smaller. The second star, Fomalhaut B, is 0.7 times the mass of the Sun and the third star, Fomalhaut C, a small red dwarf, is 0.2 times the mass of the Sun.

Fomalhaut C orbits Fomalhaut A at a distance of 2.5 light-years, or roughly half the distance from the Sun to the closest neighboring star.  It was only confirmed to be gravitationally bound to Fomalhaut A and Fomalhaut B in October of last year.

“The disk around Fomalhaut C was a complete surprise,” lead researcher Grant Kennedy of the University of Cambridge told Universe Today. “This is only the second system in which disks around two separate stars have been discovered.”

Relatively cool dust and ice particles are much brighter at long wavelengths, allowing telescopes like the Herschel Space Telescope, to pick up the excess infrared light. However, Herschel has a much poorer resolution than an optical telescope so the image of Fomalhaut C’s disk is not spatially resolved — meaning the brightness of the disk could be measured but not its structure.

Kennedy’s team’s best guess is that the disk is quite cold, around 24 degrees Kelvin and pretty small, orbiting to and extent of 10 times the distance from the Earth to the Sun. But it’s likely that it’s similar to Fomalhaut A’s disk in that it’s bright, elliptical, and slightly offset from its host star. All three characteristics suggest that gravitational perturbations may be destabilizing the cometary orbits within the disks.

“As a stellar system Fomalhaut’s gotten very interesting in the last year,” Kennedy said. With two wide companions “it’s not obvious how the configuration came about. Forming one wide companion is not so hard, but getting a second is very unlikely. So we need to come up with a new mechanism.”

Kennedy is currently working on figuring out what exactly this “new mechanism” is and he thinks the debris disk around Fomalhaut C will provide a few helpful hints. His best guess is still under construction but it’s likely that a small star is disturbing the system.

The next step will be to watch the stellar system over the next few years in order to measure their orbits exactly. With precise motions we just might be able to see what is interrupting the system.

“We think these observations will provide a good test of the theory,” Kennedy told Universe Today. They just might “solve the mystery of why the Fomalhaut system looks like it does.”

The paper has been published in the Monthly Notices of the Royal Astronomical Society and is available for download here.


Shannon Hall is an aspiring science journalist and is an editorial intern at Sky & Telescope magazine. She holds two bachelor's degrees from Whitman College in astrophysics and philosophy, and recently received her master's degree in astrophysics from the University of Wyoming.

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