Astronomers today announced a significant advance in solving the long mystery of Epsilon Aurigae, an enigmatic star that, every 27.1 years, loses half its light for almost two years. The star has mystified astronomers for nearly two centuries despite the fact that it’s easily visible to the naked eye and has been intensively observed by professional and amateur astronomers for decades.
At the American Astronomical Society meeting in Washington, DC, Donald Hoard of Caltech described recent infrared observations from NASA’s Spitzer Space Telescope and a new model that apparently, for the first time, fully ties together the mountains of available data. “What our result has provided is a big-picture solution,” said Hoard. But he was quick to add, “There are still a lot of details that need to be worked out.”
Prior to the most recent dimming, which began in August 2009, astronomers had built up a picture of the system in which the visible star, a type-F supergiant, is much more massive than the Sun. That's what its extreme luminosity (130,000 times the Sun's brightness) would suggest. Every 27.1 years, a huge dusty disk seen almost edge-on slides across the face of the star, producing the long-lasting partial eclipse. But big questions remained about the nature of the bright star, the eclipsing disk, and especially the unseen massive object or objects that must occupy the disk's center (Sky & Telescope, May 2009, page 58).
The Spitzer observations, combined with many observations at visible and ultraviolet wavelengths, provide a more complete picture of the system. The Spitzer work conclusively reveals the presence of a disk about 8 astronomical units in diameter, as expected, and also shows that it consists of relatively large particles mostly the size of sand grains, not the usual microscopically fine space dust. Far-ultraviolet observations also indicate the presence of a smaller, very hot star at the center of the disk, probably spectral type B (about three times hotter than the Sun).
The problem with this model has been that the disk's central object seems to have about the same mass as the F supergiant, and if it's a normal star with such a high mass, it should shine about equally bright. But we hardly see it at all — even though the center of the disk seems to be clear.
Hoard and his colleagues have proposed a model they say fits all the observations. The key part is that the bright F supergiant is much less massive than previously thought. It could still shine so powerfully if it is very far evolved and nearing the end of its life. In this scenario it started off with around 10 solar masses (as opposed to the 15 or 20 usually assumed for it) and has since blown off much of even that. The companion B star is then allowed to have only about 6 solar masses, and therefore shines much dimmer. In this scenario, the dark disk is not the sign of a newborn star still gathering material. The disk instead is made of material that the B star gravitationally captured from the dying primary star's wind.
The disk currently contains much less than an Earth mass, but it probably began with much more. Its original gas and microscopic dust grains have been blown out of the system, leaving only the larger grains behind. The system itself is probably about 10 million years old. Over the next thousands of years, the dying F star will puff off most of its remaining mass to form a planetary nebula.
“All of these intertwined parameters just sort of work out,” says Hoard, who adds that he previously favored another model. “I’m a convert to this model, but I am very comfortable with it.”
Arne Henden, director of the American Association of Variable Star Observers, emphasizes that the mystery of Epsilon Aurigae has not yet been solved. “We’re nearing the middle of the eclipse, and lots of interesting things will happen over the next year. There are still things about this system we don’t understand.”
Hoard and Henden both point out that the AAVSO has organized a global network of “citizen scientists” to monitor Espilon Aurigae’s changing brightness and spectrum. The massive amount of information from high-quality amateur observations (photometry and spectroscopy), as well as continued professional observations, should finally solve the mystery of Epsilon Aurigae. “If there is any time we will understand this system, it’s with this current round of observations,” says Henden.