Nature of The System
The nature of the Epsilon Aurigae system has always been unclear. It has long been known to consist of at least two components which undergo periodic eclipses with an unusual flat-bottomed dimming every 27 years. Early explanations with exceptionally large diffuse stars, black holes, and odd doughnut-shaped discs are no longer accepted. There are now two main explanations that can account for the known observed characteristics: a high mass model where the primary is a yellow supergiant of around 15 M☉; and a low mass model where the primary is about 2 M☉ and a less luminous evolved star.
Variations on the high mass model have always been popular, since the primary star is to all appearances a large supergiant star. Spectroscopically it is late F or early A with luminosity class Ia or Iab. Distance estimates consistently lead to luminosities expected for a bright supergiant. One exception is the Hipparcos parallax measurement, but the margin of error is as large as the value itself and so the derived distance is likely to be anything from 355-4167pc. The main problem with this model is the nature of the secondary, which is required to have a mass about the same as the primary, at odds with observations where it appears as a B-type main sequence star. The secondary may be a close binary involving to lower-mass main sequence stars, or a more complex system.
The low mass model, recently popularised by the Citizen Sky project, proposes that the primary is an evolved asymptotic giant branch star of 2-4M☉. This relies on distance and luminosity estimates lower than most observations. The star would be an unusually large and bright giant star for the given mass, possibly as the result of very high mass loss. To match the observed eclipse and orbital data, the secondary is a fairly normal B main sequence star of about 6M☉ embedded in a thick disc seen nearly edge on.
The orbit itself is now fairly well determined, inclined at over 87 degrees to us. The primary and the disk are nearly thirty AU apart, (in the high mass model) which is approximately the distance of the planet Neptune from the Sun.
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