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Pictures reveal binary star’s dark companion

Using an instrument created at the University of Michigan that uses “interferometry” to combine light entering four telescopes, astronomers have for the first time imaged a peculiar binary star eclipse that happens once every 27 years. Credit: Courtesy, U. Michigan

U. MICHIGAN (US)—For the first time, astronomers have directly observed the mysterious dark companion in a binary star system that has puzzled skywatchers since the 19th century.

Scientists were able to take “close-up” pictures of Epsilon Aurigae during its eclipse, which happens every 27 years.

“Seeing is believing,” says John Monnier, associate professor of astronomy at the University of Michigan.

Details of the research appear in the April 8 issue of the journal Nature.

Epsilon Aurigae is the fifth brightest star in the northern constellation Auriga. For more than 175 years, astronomers have known it is dimmer than it should be, given its mass.

They also noticed its brightness dip for more than a year every few decades. They surmised that it was a binary system in which one companion was invisible. But what type of object was the companion?

Because astronomers hadn’t observed much light from it, the prevailing theory labeled it a smaller star orbited edge-on by a thick disc of dust.

The theory held that the disc’s orbit must be in precisely the same plane as the dark object’s orbit around the brighter star, and all of this had to be occurring in the same plane as Earth’s vantage point. This would be an unlikely alignment, but it explained observations.

The new images show that this is indeed the case. A geometrically thin, dark, dense, but partially translucent cloud can be seen passing in front of Epsilon Aurigae.

“This really shows that the basic paradigm was right, despite the slim probability,” Monnier says. “It kind of blows my mind that we could capture this.

“There’s no other system like this known. On top of that, it seems to be in a rare phase of stellar life. And it happens to be so close to us. It’s extremely fortuitous.”

The disc appears much flatter than recent modeling from the Spitzer Space Telescope suggests, Monnier says. “It’s really flat as a pancake.”

Monnier led the creation of the Michigan Infra-Red Combiner (MIRC) instrument that was used to produce these images.

MIRC uses a process called “interferometry” to combine the light entering four telescopes at the CHARA array at Georgia State University and amplify it so that it seems to be coming through a device 100 times larger than the Hubble Space Telescope.

MIRC allowed astronomers to see the shape and surface characteristics of stars for the first time. Previously, stars were mere points of light even with the largest telescopes.

“Interferometry has made high resolution imaging of distant objects a reality,” explains Fabien Baron, a post-doctoral researcher in the Department of Astronomy who helped with the imaging in this study. “It most probably will solve many mysteries but also raise many new questions.”

Researchers from the University of Denver and Georgia State University contributed to the paper.

The research received funding from the National Science Foundation and the office of the dean of the College of Arts and Sciences at Georgia State University.

University of Michigan news: www.ns.umich.edu/

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