Epsilon Aurigae: A finder chart. (Photo and graphics by the Author).
One of the strangest variable stars is worth watching this spring.† Back in 2009, we alerted viewers to monitor the curious variable Epsilon Aurigae. Once every 27.06 years, this star dips nearly a magnitude in brightness down to about +3.8, markedly discernable to the naked eye. This drop lasts for over a year before Epsilon Aurigae returns to its former self. This spring should witness such an occurrence. This time around, Epsilon Aurigae has been studied in unprecedented detail by everything from the Citizen Sky project to the American Association of Variable Star Observers to none other than the Spitzer Space telescope.
So, just what IS this bizarre beast? This eclipsing variable system that once threatened to overturn models of stellar evolution is becoming better understood. The visible primary is an F2-type pulsating super-giant. The unseen eclipsing companion has been proposed to be everything from a large but extremely cool and tenuous star to a massive but shrouded object. Initially proposed in the 1970ís to be a good stellar mass black hole candidate, the picture that is emerging from the 1982-84 eclipse and recent models based on Spitzer observations is a pair of B-type stars in a tight orbit embedded in a huge ring of material that passes between our line of sight every 27+ years. The disk is massive, about 18 astronomical units in diameter, and the entire system weighs in at about 14 solar masses. This month should be the last full month of eclipse. Rather telling is the gradual decrease/increase on either end of the change in brightness, often accompanied by a brief central spike at the long mid-point. One can imagine the ragged edge of a dense torus of material with a small doughnut hole in the middle. The average temperature of the torus is about 500 Kelvin. I always think of Larry Nivenís novel The Integral Trees about an inhabited gas torus whenever I observe Epsilon Aurigae. Now is the time to catch this bizarre long period variable as it brightens, and you only need your two eyes to do it! The next eclipse wonít happen until 2038. ††††
The Astro-word for this week is Light Curve. Plot a starís brightness over time and a nice symmetrical curve may emerge. This graph is what is known as a light curve, and can tell us a great deal about the nature of a star. Some stars pulsate rhythmically, some eclipse from our line of sight and dip in a predicable manner. In this fashion we can deduce the presence of a spectroscopic companion, even if the apparent angular separation is too small to be discerned. Light curves have also been used over the past decade to identify transiting exoplanets. Are there any exo-worlds in the Epsilon Aurigae system? Itís a distinct possibility with all that material flying aroundÖ we wouldnít be surprised if a discovery was made this year as the system brightens.