[BRIEF NOTE] On Epsilon Aurigae

[rfmcdonald]

A very interesting discovery has been made in regards to the star Epsilon Aurigae. The fifth-brightest star in the constellation Auriga by Beyer designation and some two thousand light years away, for nearly two centuries astronomers have been aware that something's odd with this white giant star.

Although the star is visible to the naked eye, Johann Fritsch's 1821 observations suggest he was the first to notice that the system was a variable star. However, the system wasn't sufficiently observed until German mathematician Eduard Heis and Prussian astronomer Friedrich Wilhelm Argelander began observing it once every few years, from 1842 to 1848. Both Heis' and Argelander's data revealed that the star had become significantly dimmer by 1847, attracting the full attention of both men at that point. Epsilon Aurigae had brightened significantly, and had returned to "normal," by September of the next year. As Epsilon Aurigae attracted more attention, more and more data were compiled. The observational data revealed that Epsilon Aurigae did not just vary over a long period, but also experienced short-term variations in brightness as well. Later eclipses took place between 1874 and 1875 and, nearly thirty years later, between 1901 and 1902.

Why did Epsilon Aurigae dim? Even back in the early 20th century, the consensus was that Epsilon Aurigae's light was being blocked by another star which, in orbiting it, blocked out its light from terrestrial observers. In the middle of the presumed eclipse, light did spike upwards when the star should have been in complete blackness. But what sort of star was it that was invisible from Earth at the same time that it seemed to block the primary star's light? It turns out that Epsilon Aurigae A is orbited by a dust-shrouded bright blue star that happens to orbit Epsilon Aurigae A edge-on as seen from Earth. Andrew Moseman summarized at 80 Beats.

Epsilon Aurigae is a star system about 2,000 light years from Earth. Astronomers have been able to see it for nearly two centuries, and noticed that it dims every 27 years or so. It made sense to assume that they were dealing with a binary star system, with a larger primary star and a smaller secondary star circling around the first. But that didn’t answer all their questions. Why, for instance, did the primary star normally appear dimmer than it should? And if there is a smaller star orbiting the main star, why can’t we see it? To explain that, astronomers developed the unlikely theory that a thick disk of dust was orbiting the smaller star in the same plane as the smaller star’s orbit of the larger star [UPI].

Monnier says he first felt there was a slim probability that this explanation—a secondary star shrouded in dust—was true. To find out for sure, Monnier and his colleagues needed to catch the eclipse when the smaller star passed in front of the larger, and capture really telling images. That’s just what they did. The astronomers used the Michigan Infra-Red Combiner instrument to combine the light entering four telescopes at the CHARA array at Georgia State University. The effect is a virtual telescope that is much larger than its four constituents [Space.com].

The results, published in Nature, were staggering to Monnier. Despite the improbability of the explanation, there was indeed a “thin, dark, dense, but partially translucent cloud” passing in front of the star in the infrared pictures. 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” [Space.com].

As Wikipedia notes, "[t]he two entities eclipse each other every 27.1 years, and each eclipse lasts approximately two years. Midway through the eclipse, the system brightens slightly. This implies the presence of an opening in the center of the disk that may be filled with a single star or a second binary system. The F-type supergiant and the dust disk are nearly thirty AU apart, which is approximately the distance of the planet Neptune from the Sun."

Nico Camargo's illustration is fantastic.



Elsewhere in the Discover blogosphere, one of the co-observers commented with glee about the process of discovery.

The extraordinary thing to me about observational astronomy was always how you could put together an apparently baroque model of some complicated system, just on the basis of a precious few data points, and yet have some degree of confidence that you were on the right track. Reality is very constraining. So in some perverse sense, it almost seems like cheating to actually take pictures of the thing. Where’s the fun in that? (Of course it’s a great deal of fun.)

An age of miracles and wonders, people.