3/23/2010-- As reported by Hoard, Howell and Stencel...
(2010 to appear in the Astrophysical Journal), data have become available that span a wide spectral range, from the far-ultraviolet, through the visible range and out into the far-infrared. Because of calibration efforts, it has proven possible to combine these well calibrated data into a complete and self-consistent picture of the sources of light in epsilon Aurigae. Key to understanding this result is that interlocking requirements of distance and other constraints on F star diameter drive us to these self-consistent conclusions.
The full paper is available, free, at website:
http://arxiv.org/pdf/1003.3694v1
Clearly the F star dominates much of the visible portion of the spectrum, and for the Hipparcos distance of 625pc and the interferometrically determined diameter, 2.27 +/- 0.11 milli-arcsec, then the F star radius is 135 +/- 5 solar radii. The energy distribution is well described by a 7750K surface temperature and log gravity = 1.0 atmospheric model., as seen in Figure 1. Subtracting this well-defined F star leaves two residual signals: In the far-UV, extra flux beyond the well fit optical region, can be fitted by a B5V star, with a nominal mass equal to 5.9 solar masses, and radius of only 4 solar radii. In the infrared, clearly an excess signal is present above the F star, and this is the cold disk previously measured to have a 550K temperature. The total flux from the disk defines a luminosity and hence a total surface area. Using eclipse length as a width constraint (3.8AU radius), the implied thickness is 0.9AU, consistent with the depth of eclipse – that is, the disk does not fully cover the F star.
What does all this mean for observers? We are approaching mid-eclipse this summer, the unique time where we have a chance to see directly into the center of the disk, and perhaps verify that a hot star lurks therein. Keep watching and reporting!
Reference: Hoard, D., Howell, S. and Stencel, R., 2010 Ap.J. - in press (HHS). Taming the invisible monster: system parameters for epsilon Aurigae from the far UV to the mid-IR. http://arxiv.org/abs/1003.3694 .