Tuesday, June 3, 2008

"Real demon star" Epsilon Aurigae: next eclipse in 2009 - and "doom" mid-century?

They happen only every 27 years, and the next one is due in 2009-11: eclipses in the strange Epsilon Aurigae system that is easily visible to the naked eye - and at the same time a topic of intense research and also of another press conference during the AAS Meeting in St. Louis in the past hour. "Observations over the coming three years, when the mysterious star undergoes a once-per-generation eclipse event, may hold the secret to the extreme changes detected during the past few decades," the summary goes, wich includes the prediction - not overly stressed in the PC - that "the bright northern star called epsilon Aurigae is headed for a 'doomsday event' within a few decades." The spectrum of epsilon Aurigae looks like a normal F supergiant star with12 to 15 times the mass of the Sun. The mass ratio in the binary is close to one, implying that the companion is about 12 to 14 solar masses as well. Epsilon Aurigae exhibits eclipses like the "demon star" Algol every 27 years, which last for nearly 2 years. The next one starts in August 2009, and should run through May 2011.

The 12 to 14 solar mass second "star" is largely invisible, however! The secondary is probably a huge dark disk, not a sphere, requiring a massive central object(s) to stabilize it. Normal eclipsing binary star analysis suggests that the secondary is about 10 AU across but does not emit anywhere near the amount of light expected from a star of its size (nor is it a collapsed object: no X-rays). Another observation: Eps Aur shows low amplitude quasi-periodic light variations, similar to Cepheid variable stars. Currently its light variations are on a 67 day cycle, but - key point - were near 96 days during the last two decades: Something is accelerating in this system! At this rate, variations will become very rapid within six decades, perhaps cataclysmically so. Observations during the last eclipse furthermore suggest that the F supergiant star may be shrinking by about 1/2% per year. The duration of total eclipse (during which the F star is partially covered by the disk shaped companion) has increased by about 25% between the 1956 and 1983 eclipses. Despite this, the overall length of the total plus partial phases of eclipse - especially the time where the F star moves out from the cover of its partner - has gotten shorter!

If these trends continue, the F star will come out of eclipse (from totality) in only 1 or 2 weeks during 2011, but will still take 140 days or so to move from the beginning of the eclipse to totality next year, autumn. A 10% change in luminosity can result from a 5% change in radius or a 2.5% change in temperature. At an estimated distance of 625 pc, and assuming the F supergiant star has a typical diameter, the implied angular diameter is 3 milli-arcseconds (mas). Modern optical interferometers are capable of measuring down to fractions of 1 mas, close to that 5% change (0.15 mas) anticipated, and these measurements are underway: They could confirm that the F star's rapid evolution is causing the accelerating light changes. It should be bifurcated by the eclipse-causing disk, if indeed it is a disk, and state-of-the-art interferometers like CHARA on Mt.Wilson and MROI at Socorro should be able to monitor that. But at the same time when this high-end astronomy will be tried, people - 2009 being the International Year of Astronomy - all over the world will also be asked to monitor the beginning eclipse in a world-wide effort of "citizen science".

In other news another Nova Ophiuchi has been discovered, rather faint. • Daring astrophotographers have caught Venus 2° from the Sun as a disk, even with hints of NIR detail on it. • Images of Jupiter with and without the red spot trio on June 3 and June 1 from Europe and April 30 from Namibia. • Comet Boattini is now at 4.9 mag. and travels amongst star clusters for Southern observers. • Today's New Moon is the closest of the year. • Nice prominences on June 2 - and (better late than never) pictures of the annular eclipse 2006 in Suriname, total eclipse 2005 in the Pacific and many earlier eclipses by K. Delcourte (accompanied by lots of travel impressions). • EPOXI imaged the Earth-Moon system on May 29 as an exoplanet detection analog experiment. • How the Discovery-ISS docking looked like yesterday from New Zealand, and the ISS passing Saturn in a little movie. • And finally a strange exhibition where (spy) satellite watching meets art ...

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