Sunday, January 23, 2011

Epsilon Aurigae Solved at Last?

A strange mystery puzzled scientists for years, now it might be solved with the help of newer technologies. A group of astronomers claims a breakthrough in the long-standing mystery of Epsilon Aurigae. This 3rd-magnitude star, a type-F super giant 130,000 times brighter than the Sun, loses half its light for almost two years every 27.1 years when a nearly opaque dust disk, seen edge-on, slides across its face.

The problem is that the disk must have a massive body in its middle, but nothing is seen. Astronomers are quite sure that the disk's central object is at least as massive as the F stars, so it ought to shine at least about as brightly. It can't be totally hidden by the disk itself; the disk is open in the middle, like a flattened doughnut tilted slightly from edge on. We know this because around mid-eclipse, we see part of the F star through the opening. So what is this unseen thing in plain sight? Many ideas have been proposed and ruled out. (No, it can't be a black hole.)

The key that unlocks the mystery, says Donald Hoard of Caltech, is to not assume that the disk means Epsilon Aurigae is a young star system that's still forming. If instead it's nearing the end of its life, the bright star can have a much lower mass than has been presumed for it. Even a star with just 2 solar masses can shine with super giant brilliance in its death throes. This would allow the disk's central object to have only 6 solar masses, also much lower than formerly presumed. So it could be a normal, garden-variety type-B star with only a couple hundred times the Sun's luminosity bright by ordinary standards, but about 1,000 times dimmer than the super giant and lost in its glare.

The entire observed energy output from the Epsilon Aurigae system, from the far ultraviolet to the far infrared (colored line and dots), is now fully accounted for by just three objects: the bright F super giant star, a single hot B star about 1,000 times dimmer, and the enormous,warm dust disk glowing in the mid- and far infrared.

Epsilon Aurigae began dimming again on schedule last August and reached its minimum brightness in December, where it will more or less remain until March2011. The difference in Auriga's familiar pattern (above) has become plain to careful sky watchers. NASA's Spitzer Space Telescope has confirmed the presence and size of the warm dust disk: it's 8 astronomical units wide. Far ultraviolet satellites have found traces of B-star light coming from some wherein the system. Hoard and his colleagues propose that the disk is material that the B star has gravitationally captured from the dying primary's thick wind.

"All of these intertwined parameters just sort of work out," says Hoard. He estimates that the system is 10 million years old. If so, we're catching it at a lucky time(which may be why it’s unique). Over the next thousands of years, the dying F star will puff off most of its remaining mass to form a planetary nebula. But the book is far from closed. Intensive observations continue worldwide,including by amateurs doing long-term photometry and spectroscopy.

Normally, Epsilon Aurigae is slightly the brightest of the three "Kids" stars south of Capella. But currently it’s only a match for Zeta Aurigae, and the change is fairly plain to the eye. Stars are labeled with their visual magnitudes.

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