Answering a 190-year old astronomical question

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Epsilon Aurigae was first given serious, systematic, scientificscrutiny in 1821. Early modern astronomers correctly classified it asan eclipsing binary variable star, with an invisible partner that will periodically dim thelight as it eclipses the main star from the perspective of Earth. This happens every 27 years, and Epsilon Aurigae's apparent brightness drops for a period of more than a year. Thenature of this partner has remained a mystery, even though we've been observing the star for nearly two centuries.

Over the years, different ideas have come and gone. Early hypotheses as to the nature of Epsilon Aurigae was that it is a F-type supergiant star with a massof over 15 solar masses. For the darker companion, people have proposed that it is an infrared star, a blackhole complete with accretion disk, or (most recently) a disk of opaquematerial orbiting the companion star. The difficulty with the most recent interpretation is the improbabilityof the orbits.

For the latter case to be correct, then theorbit of the disk around the darker companion star would have to be in the same planeas the orbit of the darker object (companion star) around Epsilon Aurigae,which would in turn have to be the exact same plane as Earth's vantagepoint in order to produce the sequence of events we observe here onEarth. New, direct, observations show this is indeed the case—look at enough stars, and you'll apparently see the improbable.(Insert something about a large number of monkeys randomly bangingawayon a large number of typewriters knocking out this exact Nobel Intentarticle given enough time.)

New observations of the system are reported in a letter in last week'sedition of Naturebased on data collected using Georgia State University's Center for HighAngular Resolution Astronomy interferometer with the MichiganInfra-Red Combiner. They've produced a series of direct images of the2009 Epsilon Aurigae eclipse (along with a snapshot from 2008 to use as a baseline).

Combining this data and some that is in press fromother research groups, the authors report that the main star has a massof 3.63±0.68 solar masses (much less massive than earlier estimates), its dark companion has a mass of5.9±0.1 solar masses. The disk of dust that orbits around the companion has anegligible mass, 0.07 time the mass of the Earth. From the images, theyare able to describe the disk, improbable orbit and all, as a cylinder with a radius of3.81±0.01 AU (the distance from the Earth to the Sun) and a heightof 0.76±0.02 AU. The authors conclude that the disk is anoptically thick but geometrically thin, suggesting it is a debrisdisk as opposed to a young stellar object.

Nature,2010. DOI: 10.1038/nature08968(AboutDOIs).

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