EPS CRA (Epsilon Coronae Australis). An obscure star that makes part of the northern curve of Corona Australis, the Southern Crown, fifth magnitude (4.83) Epsilon Coronae Australis (no surprise, of no proper name) turns out to be among the brightest of its bizarre class. At a distance of 98 light years, this class F (F2) dwarf, with a temperature of 7000 Kelvin, shines with the light of only 8 Suns, the radius (calculated from temperature and luminosity) 1.9 solar, the mass 1.6 solar. However, Epsilon CrA is not one star, but two, locked in the tightest of embraces. Over the course of just one night, it can be seen to vary continuously and regularly by about a quarter of a magnitude over a period of 7 hours, up and down, with no halting. Epsilon is an eclipsing binary with two very similar eclipses that take place within an orbital period of 0.5914264 days, as first a faint star passes in front of the bright one that gives us 95 or so percent of the light, and then the bright one passes in front of the fainter. The light curve, the graph of magnitude against time, tells us immediately that Eps CrA is the brightest southern-hemisphere example of a "contact binary," otherwise known after the prototype as a "W Ursae Majoris star." Its only rival in the northern hemisphere is 44 Bootis. The members of this system are so close together that they are subject to fierce tidal distortion. Both stars fill their "Roche lobes," three-dimensional teardrop-shaped tidal surfaces at which the gravitational forces are balanced between the pair. At the inner points of the teardrops, the stars touch. Part of the variation we see comes from the distortion itself as the brighter star presents constantly changing cross-sections to us. Since the spectra of both stars are observed, we can determine orbital velocities. Combination of these with the light curve yields the stellar parameters. The
Above, Epsilon CrA's brightness varies continuously as each star passes in front of the other, indictative of a contact binary. To the right, the radial velocity curves of the two stars differ enormously, showing that the principal star (the one that shows the least movement) is 8 times more massive than the secondary. Light curve from S. M. Rucinski in the Publications of the Astronomical Society of the Pacific; velocity curves from K.-D. Goecking and H. W. Duerbeck in Astronomy and Astrophysics.
stars are only 0.0172 Astronomical Units, just 2.9 million kilometers, apart, the plnae of the orbit tilted through 72 degrees relative to the plane of the sky. The brighter and fainter have respective radii of 2.20 and 0.85 solar radii and masses of 1.72 and 0.22 solar masses (very close to the values derived from luminosity and temperature), giving Eps CrA one of the highest mass ratios known among contact binaries. The "radii" are misleading and do not add up to contact because the stars are so distorted. The rotational period determined from the spectrum is in close agreement with the orbital period. The stars spin so fast that they produce considerable magnetic activity and dark starspots that also add to the variation as they swing in and out of view. Such systems are not well understood. In spite of its much lower mass, the smaller star has the same temperature as the larger one, an effect called the "contact binary paradox," the more massive star somehow feeding energy into the less massive. As strange as these systems are, they are not that uncommon. As many as one percent of the solar type stars may be in such a situation. Will they merge into one? Nobody knows. Not content with such oddness, Epsilon CrA adds to it by being a relatively high velocity star, moving at 60 kilometers per second relative to the Sun, over three times faster than normal, showing that it is not a regular member of the solar neighborhood.
Written by Jim Kaler. Return to STARS.