Two minute warning...

If you've tried to observe epsilon Aurigae lately, it is quickly sinking into the northwest at dusk, along with Orion and other winter constellations.  The star is at solar conjunction during early June, making observations more challenging over the coming several months.  To stretch the game analogy, it's essentially half-time - in terms off the eclipse schedule.  Naturally, this is just when the fabled mid-eclipse brightening is forecast to happen, suggestive of a central clearing in the midst of the dark disk.  Given the developments of the past months and the coming half-time show, it seems timely to review what's been learned, and outline some of the outstanding questions that further observations can help address. 

As we approach mid-eclipse, the following facts are emerging that challenge understanding of the theory of this star's evolution and its state of binary interaction:

--the F star is wildly over-luminous for its inferred low mass;

--the existence of a disk appears certain, but details of its overall shape, composition, age and evolutionary status remain to be defined;

--the disk appears to possess an extended atmosphere, as detected in the neutral metals (potassium and sodium plus other low temperature lines), and this gas may overfill the Roche lobe of the disk and its central star, perhaps encompassing the binary;

--the disk appears to have internal structure consistent with rings or tidally-induced spiral density waves, based on neutral potassium variations during ingress, plus polarimetry from the last eclipse;

--ideally, mid-eclipse will enable an interferometric view of the center of the disk and its contents; 

--ideally, egress will provide further spectroscopic evidence for the nature of dust in the disk, in terms of sublimation production of CO (and perhaps ices), due to differential heating, and reveal composition and dynamics.

For all these reasons and more, we urge all observers to keep up the wonderful effort to provide as much detail in these coming phases as has been seen for ingress.  Those observers at higher latitudes (Canada, UK, northern Europe, Scandanavia, Russia, Japan...) and those at higher altitudes (mountaintops, ISS...) have the best chance of obtaining validated data during these solar conjunction months.  As a reminder, mid-eclipse is forecast to occur in early August, and eclipse to persist until spring 2011.  Thus, make the most of the coming fall/winter observing opportunities (or wait until next round, 2036).  I'm scheduled to report on these details at the Symposium of the Society of Astronomical Sciences this week (www.socastrosci.org), and will relate outcomes of discussions held there in coming blog posts, if fire, flood, earthquake and volcanos don't intrude.  Happy observing!