EPS EQU (Epsilon Equulei). From the nineteenth century, Smythe and Chambers tell us of "a delicate triple star... A white, B lilac...with a bright star following at a distance...", the charming violet color an artifact of proximity, which, with stars of different brightnesses, fools the eye. In fact both are white. And the writers were here more than half right, as fifth magnitude (5.23) Epsilon Equulei (also Flamsteed 1, in Equuleus, the Little Horse, a tiny constellation that lies to the southwest of its bigger neighbor, Pegasus) seems to be quintuple. Smythe and Chambers' brighter AB pair of stars are tight together, less than a second of arc apart, both sixth magnitude (Eps A and B 5.8 and 6.2) class F (F5 and F7) subgiants 176 (give or take 13) light years away that are close to giving up core hydrogen fusion, if they have not done so already. Temperatures are not well known. A combination of a measurement and that expected from the class yields 6500 Kelvin for "A," the class alone giving 6300 for "B." Luminosities come in at 11.1 and 7.8 times that of the Sun, radii at 2.6 and 2.3 solar, theory giving masses of 1.6 and perhaps 1.55 (depending on the exact state of subgiant evolution), and an age of 2.2 billion years. An equatorial rotation speed of 42 kilometers per second, which probably applies to Eps A, results in a rotation period of under 3.1 days.
Equulei Epsilon Equulei B orbits Eps Equ A (at the cross) almost edge-on with a period of 101.5 years on a quite eccentric orbit that takes them from as far as 60 U apart to as close as 10 AU (the eccentricity exaggerated by the orbital tilt). A little bit more tilt and the two would eclipse each other. In reality the pair orbit each other around a common center of mass. The scale is in seconds of arc; note how close the stars are. (W. I. Hartkopf and B. D. Mason, Sixth Catalog of Orbits of Visual Binary Stars, US Naval Observatory Double Star Catalog, 2006.)
The pair orbit each other every 101.485 years at an average separation of 35 Astronomical Units, a substantial eccentricity taking them between 60 and 10 AU apart. They were last closest together in 1920 and will be again in 2121, the high orbital tilt almost making an eclipsing double out of them. Kepler's Laws yield a total mass of 4.2 Suns, greater than the 3.2 Suns found from luminosity and temperature. The difference may be caused by a spectroscopic companion to Eps Eql A ("Ab"), making it a roughly solar mass star, which would slightly reduce the luminosity of "A." Ab's short period of a mere 2.03 days suggests an orbital size of just 0.04 AU, around 10 percent Mercury's distance from the Sun. But the mass of Ab is very sensitive to the A-B separation; shrinking that orbital size to 32 AU makes the it a very low mass star. Then 6 seconds of arc from the inner triple, we find Smythe and Chambers' third star, seventh magnitude (7.1) Eps C, whose brightness would make it another class F5 star of a bit over a solar mass. A separation from the inner trio of at least 665 AU leads to an orbital period of at least 7500 years. Finally, nearly 80 seconds of arc from Eps A and B (and Ab) lies thirteenth magnitude Epsilon D, which would -- if a real companion -- be a class K8 dwarf that would orbit at a distance of at least 4300 AU every 125,000 or more years. Are "C" and "D" really part of the system? They seem to be tracking AB within roughly a second of arc over centuries (C) and decades (D) of time. Given that AB is moving at nearly 20 seconds of arc per century against the distant background, they do indeed seem to belong. The real mystery involves the short-period companion to A, whose mass will slowly be refined by better observations of the inner triple.
Written by Jim Kaler 10/14/11. Return to STARS.