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January 18, 2012

Scientists discover Saturn-like ring system eclipsing sun-like star

graphic of dust rings with sun

A team of astrophysicists has discovered a Saturn-like ring system in the constellation Centaurus.

Led by Eric Mamajek, assistant professor of physics and astronomy at Rochester and an associate astronomer at the Cerro Tololo Inter-American Observatory, scientists used data from the international SuperWASP (Wide Angle Search for Planets) and All Sky Automated Survey (ASAS) project to study the light curves of young sun-like stars in the Scorpius-Centaurus association—the nearest region of recent massive star formation to the sun.

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Imagine yourself sitting in a park on a sunny afternoon and a softball passes between you and the sun. The intensity of light from the sun would appear to weaken for just a moment. Then a bird flies by, causing the intensity of the sunlight to again weaken—more or less than it did for the softball, depending on the size of the bird and how long it took to pass. That's the principle that allowed the researchers to discover a cosmic ring system.

A light curve is a graph of light intensity over time, and one star in particular showed dramatic changes during a 54-day period in early 2007. Rochester graduate student Mark Pecaut and Mamajek discovered the unusual eclipse in December 2010.
"When I first saw the light curve, I knew we had found a very weird and unique object. After we ruled out the eclipse being due to a spherical star or a circumstellar disk passing in front of the star, I realized that the only plausible explanation was some sort of dust ring system orbiting a smaller companion—basically a 'Saturn on steroids,'" says Mamajek.

If a spherical object merely passed in front of the star, the intensity of the light would gradually dim and reach a low point before gradually increasing. That was not the case with the star identified as 1SWASP J140747.93-394542.6.

As reported in the Astronomical Journal, the Rochester team discovered a long, deep, and complex eclipse event with significant on-and-off dimming. At the deepest parts of the eclipse, at least 95 percent of the light from the star was being blocked by dust.
The shape of the light curve was very similar to that of a well-researched star (EE Cephei), suggesting similar traits in the companion objects. However EE Cephei differs in that it appears to be a thick protoplanetary disk passing in front a massive, hot star.

"We suspect this new star is being eclipsed by a low-mass object with an orbiting disk that has multiple thin rings of dust debris," says Mamajek. The star is similar in mass to the sun but is much younger—about 16 million years old or 1/300th the age of the solar system—and it lies about 420 light years away.

“This marks the first time astronomers have detected an extrasolar ring system transiting a sun-like star, and the first system of discrete, thin, dust rings detected around a very low-mass object outside of our solar system,” says Mamajek. “But many questions remain about what exactly has been discovered.”

He says the object at the center of the ring system could be a very low-mass star, brown dwarf, or planet. The answer lies in the object’s mass.

Along with the central object, Mamajek is interested in what is taking place in the two pronounced gaps located between the rings. Gaps usually indicate the presence of objects with enough mass to gravitationally sculpt the ring edges, and Mamajek thinks his team could be either observing the late stages of planet formation if the transiting object is a star or brown dwarf, or possibly moon formation if the transiting object is a giant planet.

If the dusty rings are similar to Saturn’s in terms of their mass per optical depth, then the total mass of the rings is only on the order of the mass of Earth’s moon. The orbital radius of the outermost ring is tens of millions of kilometers, so the mass and size of the ring systems is substantially heftier than Saturn’s ring system.

In the paper, the four rings detected thus far have been dubbed “Rochester,” “Sutherland,” “Campanas,” and “Tololo” after the sites where the eclipsed star was first detected and analyzed.

With several questions still to answer, Mamajek considers the paper to be a progress report. He expects it will take at least a couple more years to piece everything together.

“Follow-up observations of such eclipses may provide our first observational constraints on the formation and early evolution of moons around gas giant planets,” he says.

The research was conducted by collaborators at Rochester, Leiden University in the Netherlands, and University of St. Andrews in Scotland.

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