First Contact

What is meant by first contact?

Basically it's the start of the eclipse. The point where the eclipsing body first touches the primary star's image and starts a decrease in the primary star's brightness as seen from Earth. But it's not so simple. With epsilon Aurigae, first contact appears to be wavelength dependent. This means that the longer (red and visual bands) wavelengths will start to show an eclipse before the shorter (blue and ultraviolet bands) wavelengths. 

In addition, because epsilon Aurigae is known to vary a fair amount out-of-eclipse, knowing exactly when the eclipse is starting (first contact) is not easy. One approach, and the one used for the Campaign predictions, is to determine the average magnitude for a given band out-of-eclipse. Next, after the eclipse is well underway there will be an ingress curve or line showing the system getting fainter with time. This should be a fairly straight line. The slope of that line is important as it can be used along with an intersection with the average out-of-eclipse value to determine the first contact point, also for second contact..

Unlike many eclipsing binary star systems where there are dozen, hundreds or even thousands of eclipse observations, because of epsilon Aurigae's long period of 27.1 years there are only a handful of eclipse observations and most of those were prior to many significant developments in equipment and techniques.

Another variable for predicting first contact is that the precise time between first contacts appears to be changing for each eclipse. The 2009/2011 eclipse will add another layer of data to these predictions.

For this current eclipse the V band prediction for first contact is 30 July 2009. RI and JH bands are most likely to be days or weeks earlier. We have no data from previous eclipses so no predictions for those bands. The B band is predicted for 11 August 2009 and the U band 21 August 2009. 

Other contact points and mid-eclipse timing have similar problems. It  will be interesting to see how these times compare with current predictions and with past eclipses. Much will be learned from the new data. Perhaps some surprises and enlightenment.