EW Boo - An Eclipsing Binary Star

In the summer of 2008, Dr. Matthew Beaky of Truman State University, Kirksville, MO asked me to make some measurements on the star known as EW Boo (HIP 73612, V=10.27 m). The star had been detected by the Hipparcos satellite to be an eclipsing binary of the Algol type. It also was reported to be of spectral class A0 which stars often show pulsations similar to delta Scuti type stars. An Internet literature search produced several timings of the primary eclipse minimum with an orbital period of some 0.906336 days (21.75 hours) but no complete light curve could be found. Accordingly, I set out to measure the changes in brightness of this star in V, B and Ic bands at the ASEM observatory in Moscow Mills, Missouri. I measured several minima and confirmed the orbital period to a difference of less than 0.00001 day (0.7 seconds). I also measured a (more or less) compete light curve as represented below by a phase diagram.

The red crosses are my measured data (some 2,858 points over several weeks). The blue dots are calculated values from a model discussed below. The secondary minimum is well shown but of more interest is the definite oscillation in the light curve between 0.10 and 0.30 phase. These appear to be classic delta Scuti oscillations. As this phase curve is a composite of many detached light curves over several weeks, that the oscillations didn't get thoroughly mixed up and unrecognizable suggests that the oscillations are in phase with the orbital parameters.

The light curve was modeled using Binary Maker 3 with some data from a recently discovered paper by Soydugan, et al, (1) in which the temperature of the primary star was reported to by 8179 K (spectral type A6V). They also indicated a mass ratio of 0.217. I came up with a orbital tilt angle to our line of sight at some 75 deg (partial eclipses) and a temperature for the smaller star of some 5000K. The calculated model is shown below.

Note that the smaller star is stretched out of spherical by the gravity of the much larger star.

The model also predicts the relative radial velocities for the two stars and some spectrographic data would help with refining the model and determining some actual physical dimensions for the system.