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In the news this month... the nature of the eclipsing binary system epsilon Aurigae

The eclipsing binary epsilon Aurigae
The eclipsing binary epsilon Aurigae CREDIT: NASA/JPL-Caltech/R. Hurt (SSC/Caltech)
Many stars vary in brightness, sometimes due to changes within the star itself such as novae or Cepheid variables, others because of external factors. One well-known variable star is epsilon Aurigae, an F-type supergiant in the constellation of Auriga, located at an estimated distance of 625 parsecs (2,100 light years). Since its variable nature was discovered in the 1820s, the star has been seen to fade in brightness every 27.1 years. During these eighteen-month-long eclipses, the brightness of the star fades to around 50 per cent of its normal magnitude. While the variability of the system has been well-studied, the exact physical nature of the eclipsing companion is less certain as it has remained undetected, and many models have been put forward to explain the unusual nature of the system. Observations of epsion Aurigae show that the star and its darker companion have a similar mass which, until recently, was thought to be around 15 times the mass of the Sun. More recent observations have shown that the supergiant star has a much lower mass of between two and three solar masses, and that the companion may be a single B5V-type star embedded within a disk of opaque material.

Now, using the CHARA interferometer, an array of infrared telescopes located on Mount Wilson in California, a team led by Brian Kloppenborg from the University of Denver have for the first time imaged the eclipsing object as it transits the disk of the star. This is the first time a spatially resolved observation of an eclipsing binary has been made. Their observations show that the eclipsing object is an opaque disk of dust, tilted to our line of sight by an estimated 84 degrees. From the motion of the disk between two observations carried out in November and December 2009, the team infer that the companion object is more massive than the visible F-type supergiant. Assuming the B-type star within the disk has a typical mass of 5.9 solar masses, the researchers calculate a mass of 3.6 solar masses for the F-type supergiant. They also calculate that if the disk is composed entirely of dust, then its mass is less than 10 per cent of the Earth's.

While the nature of the disk is now clearer, there are still several unanswered questions which remain. The model that best fits the data is of a geometrically thin disk tilted to our line of sight, rather than a thick disk seen edge on. However, the fact that it is opaque suggests that its nature is more like a debris disk than a dusty accretion disk around a young stellar object. The tilted disk model also predicts a central hole which should cause a mid-eclipse brightening of the F-type star. Observers the world-over will continue to monitor the system during the eclipse, and the data should help build up a profile of the disk and constrain the evolutionary history of the system.

This blog post is a news story from the Jodcast, aired in the May 2010 edition.

Kloppenborg, B., Stencel, R., Monnier, J., Schaefer, G., Zhao, M., Baron, F., McAlister, H., ten Brummelaar, T., Che, X., Farrington, C., Pedretti, E., Sallave-Goldfinger, P., Sturmann, J., Sturmann, L., Thureau, N., Turner, N., & Carroll, S. (2010). Infrared images of the transiting disk in the ε Aurigae system Nature, 464 (7290), 870-872 DOI: 10.1038/nature08968

Posted by Megan on Monday 31st May 2010 (12:19 UTC) | Add a comment | Permalink


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