Epsilon Aurigae’s companion moves into the light

The eclipse of Epsilon Aurigae is an odd one. Every 27 years or so for a period of 18 months, the brightness of the star dips. It gets dimmer and dimmer, then lights up a bit and then gets dimmer again before slowly brightening. The behaviour was first recognised 180 years ago and during the last eclipse, around 27 years ago, a model of what was happening was devised. This model suggests there is a companion object to the star in orbit of it. This object has a dusty disc that is slightly tilted from our point of view, just enough that it and the hole or glowing thing in the middle, can be seen.

During the present eclipse, the calculations pertaining to this model can finally be put to the test – does the eclipse really behave as scientists believe it should? Measurements can be performed even by members of the public through the citizen sky project (indeed the first indications of the thinnest bit of the dust cloud arriving were spotted through Robin Leadbeater’s spectroscope), but using the best tools available to astronomers, some interesting things have already been seen. Like the very tip of the disc looming before the light of Epsilon Aurigae.

The star is close and big enough to be observed as a disc, one of only a dozen or so that can be. This means that by combining the light of six telescopes (a process known as interferometry) to improve on their resolution and sensitivity, images can be taken of events happening during the eclipse for the first time. This was not available to the instruments and scientists of three decades ago and took place at the Center for High Angular Resolution Astronomy (CHARA) using the Michigan Infrared Combiner (MIRC), which allows the combination of high frequency signals such as infrared and visible light. The result of such a technique is a resolution equivalent to a 330m telescope (the distance between the farthest telescopes of the array), though the sensitivity is equivalent only to the sum of the areas of the involved telescopes.

Epsilon Aurigae is a naked eye F type star with a mass of 3.6 times that of the Sun. The disc is thought to be twice the diameter of the orbit of Jupiter (2.4 billion km) and as thick as the diameter of the orbit of Earth. It would appear that the disc hides a star 150 times the size of the Sun and 5.9 times its mass, a B5V star. The team behind the observing project took images over the course of a month and put them into a video to show the shadow of this companion creeping across the face of the star. Observations will continue throughout the eighteen month eclipse, with the hope that the hole (if it is there) will be seen. The disc has already been seen to be translucent – it allows light from the star behind to pass through it, allowing the absorption spectrum (fingerprints of atoms and molecules in the light they absorb) to be observed. From that, the astronomers have deduced the entire enormous disc has only the mass of one tenth of the Earth.

Image Credit: John D. Monnier, University of Michigan, Nature 8 April 2010

Image Credit: John D. Monnier, University of Michigan, Nature 8 April 2010

The press release from the observing team is here.

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