Astronomers Image Mysterious Dark Object That Eclipses Epsilon Aurigae

Epsilon Aurigae has baffled astronomers since the 1800’s, but new images are providing insight into this very unusual eclipsing binary star. While eclipsing binary stars aren’t unique in themselves, the way this star fades and then regains its brightness is inimitable and has not been fully understood, even after over 175 years of study. One theory has been that a large opaque disk seen nearly edge-on eclipses the primary star. The new images from an instrument developed at the University of Michigan appear to confirm that theory. “It kind of blows my mind that we could capture this,” said John Monnier from U-M. “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.”

Epsilon Aurigae has a two-year-long eclipse that occurs every 27 years. The current eclipse started in August 2009 and amateur and professional astronomers have taken this opportunity to train as many telescopes on the event as possible.

Monnier led the development of the Michigan Infra-Red Combiner (MIRC) instrument, which uses 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 eclipsing object for the first time.

The object that eclipses the primary star is dark — almost invisible — and is only seen as it passes in front of Epsilon Aurigae, the fifth brightest star in the northern constellation Auriga. Because astronomers hadn’t observed much light from it, one theory is the object was a stellar mass black hole. But the prevailing theory labeled it a smaller star orbited edge-on by a thick disk of dust. The theory held that the disk’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. As unlikely as this alignment would be, it explained the 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 said, and the disk appears much flatter than recent modeling from the Spitzer Space Telescope suggests. “It’s really flat as a pancake,” he said.
[/caption]

While the “movie” of the disk passing in front of the star looks eerily like Saturn’s rings, Monnier doesn’t think the object is like a ring system.

“Ring systems are generally (always) quite sparsely populated and not optically thick,” Monnier said in an email to Universe Today. “Also ring systems have virtually no gas and settle into *extremely* thin layers. Both of these facts make it highly unlikley that the dust Eps Aur is in a “ring” because it wouldn’t be able to completely absorb so much of the star light during eclipse. That said, we don’t know much about the distribution — there might be a bit of a central hole as indicated by brightening of the star during mid-eclipse seen in the past.”

As to why this object is so dark, Monnier said, “At this epoch we are seeing the back side that can’t do any reflecting. We would expect some light to scatter off at other times in the orbit and would be worth looking for but requires very high angular resolution and high dynamic range. Note that the disk is not completely dark — the infrared glow of the cool dust grains have been seen in the 1980s and most recently in a Spitzer space telescope paper by Hoard et al.” (See the paper, “Taming the Invisible Monster: System Parameter Constraints for Epsilon Aurigae from the Far-Ultraviolet to the Mid-Infrared.”

MIRC has also 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,” said Fabien Baron, a post-doctoral researcher at U-M who helped with the imaging in this study. “It most probably will solve many mysteries but also raise many new questions.”

The new findings will be published in the April 8 edition of Nature. Researchers from the University of Denver and Georgia State University also contributed to the research.

Sources: EurekAlert, email exchange with John Monnier

Epsilon Aurigae Eclipse Mystery Solved with Your Help

If you’ve been helping out with the Citizen Sky project to monitor Epsilon Aurigae, then congratulations – the first of the results are in! Donald W. Hoard, a research scientist at Caltech announced the findings at the American Astronomical Society meeting in Washington, D.C. this morning. We invited our readers to participate in monitoring the star in August of 2009, and combined with observations from Spitzer, a 200-year old mystery has potentially been solved.

Epsilon Aurigae is a bright star in the constellation Auriga. It began to dim in brightness last August, which it does every 27 years. The star dims for over 2 years, with a slight brightening in the middle of the eclipse, making it the longest known orbital period for a stellar eclipse. The Citizen Sky project invited professional and amateur astronomers alike to aid in the observation of the star during this eclipse.

What exactly passes in front of the star was a mystery, though it was thought that a large disk of material with two stars orbiting tightly in the center is the cause of the eclipse. The disk itself is pretty huge – on the order of 8 astronomical units. There is a slight brightening during the middle of the eclipse that led astronomers to believe there is a hole created by the two stars in the center.

“If [Epsilon Aurigae] were an F star, with about 20 times the mass of the Sun…a single B star at the center of the disk doesn’t have enough mass to explain the orbital dynamics,” Hoard said. Other possibilities proposed were the presence of a black hole at the center of the disk, but there were no telltale X-rays coming from the system that would show a black hole was heating up matter in the disk.

Through observations by astronomers that participated in the project, as well as observations made by the Spitzer space telescope, a major revision of the properties of Epsilon Aurigae itself were in order.

“What we were most pleased to find an answer to… was that the results strongly tip towards a 2 solar-mass dying star. Sometime in the next few thousand years it will emerge as a planetary nebula,” Hoard said.

This means that instead of being a 20 solar-mass F-star supergiant, Epsilon Aurigae is in fact a 2 solar-mass F-star which is in the last stages of its life, and thus giant in size – about 300 Suns across. This, combined with a single B-star of about 5.9 solar masses at the center of the disk that orbits Epsilon Aurigae fit the observations very well, Hoard said.

Arne Henden of the American Association of Variable Star Observers (AAVSO), commenting on Hoard’s presentation at the press conference, said “Don says that we solved it, and I disagree. We need to determine the nature of the dusty disk that is involved – these are things that you see around young stellar objects, not older stellar objects.”

Hoard said that there was a curious property of the disk in that it was composed of larger grains of dust – more like grains of sand than microscopic dust motes.

“The observations that are being made by Citizen Sky project…will hopefully help answer this by providing answers about the composition of the disk and the temperature zones as the eclipse continues. We have these results in large part due to the effort of this huge group of citizen astronomers that are observing Epsilon Aurigae,” he said.

Epsilon Aurigae is still undergoing its eclipse, though the first phase ended right around the New Years Eve 2009. It will continue to be dim until early 2011, when it will begin to brighten again. There is still a lot to be answered about this system, and your help is needed, so keep (or start) observing and reporting! For more information on how to do so, visit Citizen Sky.

Source: AAS press conference on USTREAM, Citizen Sky press release

Help Solve the Mystery of Epsilon Aurigae with Citizen Sky

[/caption]
We’ve written about Epsilon Aurigae before, but this mysterious star is just now beginning to dim, so we wanted to remind everyone that they can be involved in real science and help solve a mystery! The variable star Epsilon Aurigae is now beginning its puzzling transformation that happens every 27 years. “That means the last time Epsilon Aurigae had an eclipse we were all rockin’ big hair and sporting shoulder pads in all of our clothes,” said Rebecca Turner, coordinator for a special project for the IYA organized by the American Association of Variable Star Observers (AAVSO). Astronomers can’t figure out why this mysterious star dims on a regular basis, so to help solve the mystery they are calling for assistance from thousands of citizen scientists.

That means you can help contribute to real astronomical research!

Since its discovery in 1821, the supergiant star Epsilon Aurigae has dipped in brightness like clockwork every 27.1 years as it is eclipsed by a very large companion object. But based on the shape of the lightcurve and the spectra that have taken of the system, astronomers can’t figure out what exactly what kind of object is eclipsing the star. Another strange feature of the lightcurve is that there is a slight brightening in the middle of the eclipse.

“The leading theory is that the secondary is surrounded by a large opaque disk,” said Turner, on the July 7 episode of the 365 Days of Astronomy podcast. “This would explain why light from the secondary doesn’t seem to be showing up in spectra. The disk seems to have a hole in the center, which would account for the mid-eclipse brightening. Current thinking is that perhaps the center of the disk is home to 2 less luminous, tightly orbiting stars. This tight orbit could create what astronomers are calling a gravitational eggbeater effect – creating that hole in the disk. Theories of a large planet falling into the stars at the center of the disk have also been introduced recently.”

Sky map of Epsilon Aurigae
Sky map of Epsilon Aurigae

Epsilon Aurigae is a bright star that can be seen with the unaided eye even in bright urban areas of the Northern Hemisphere from fall to spring. But it is also too bright for most professional telescopes to observe, so this is where the public comes in.

“It’s not just amateurs with fancy telescopes and CCDs or photoelectric photometers that are needed for this experiment,” said AAVSO’s Mike Simonsen. “People with just their eyes or a pair of binoculars can contribute to understanding this weird star by observing epsilon Aurigae over the next two years and reporting their observations to AAVSO.”

A diagram of the most popular model of the epsilon Aurigae system, by Jeff Hopkins:
A diagram of the most popular model of the epsilon Aurigae system, by Jeff Hopkins:

For this project, a new website has been launched called “Citizen Sky”, and all you need are a good pair of eyes, and a finder chart, which can be found on the website. No previous astronomical experience is necessary.

The project is supported by a three-year grant from the National Science Foundation to recruit, train, and coordinate public participation in this project. What makes this project different from previous citizen science projects is its emphasis on participation in the full scientific method. Participants are not being asked simply to collect data. They will also be trained to analyze data, create and test their own hypotheses, and to write papers for publication in professional astronomy journals. Participants can work alone on all phases of the project or they can focus on one stage and team up with others.

Epsilon Aurigae is just now beginning to dim. It will remain faint during all of 2010 before slowly regaining its normal brightness by the summer of 2011.

The lead astronomer for this project is Dr. Robert Stencel, the William Herschel Womble Professor of Astronomy at Denver University. Dr. Bob, as the amateur astronomy community knows him, studied the last event in 1982-84 while working at NASA. “This is truly an amazing star system. It contains both a supergiant star and a mysterious companion. If the supergiant was in our solar system, its diameter would extend to Earth, engulfing us,” Stencel said. “The companion only makes its presence known every 27 years and is a type of ‘dark matter’ in that we indirectly detect its presence but don’t know what it is.

“To make things even more fun, we also have some evidence of a substantial mass, perhaps a large planet, spiraling into the mysterious dark companion object. Observations during the upcoming eclipse will be key to understanding this and predicting what will happen if the putative planet does eventually fall into the star,” Dr. Bob added.

Here’s a video with Rebecca Turner explaining more about Citizen Sky.

For more on Epsilon Aurigae, see this page from AAVSO
Citizen Sky

Sources: 365 Days of Astronomy,