EPS ARI (Epsilon Arietis). Double stars run the gamut of
separations and brightness differences, from components that nearly
touch each other to "fragile" cohabitations such as the tie between
Proxima Centauri and the far brighter
and more massive binary (Alpha
Centauri) that it orbits; from nearly equally bright companions
to those like Sirius, where one is ten
thousand or more times brighter than the other. Epsilon Arietis,
a fifth magnitude (4.60) binary, which lies well to the east of the
classic figure of Aries (the Ram), is of
the visually-close-nearly-equal variety. Of the same class, both
white type A (A3) hydrogen-fusing dwarfs, fifth magnitude (5.17)
Epsilon A (the eastern of the two) is just 0.4 magnitudes brighter
than Epsilon B (5.57). In the nineteenth century, Smythe and
Chambers called them "pale yellow and whitish," the coloration a
contrast effect. F.G.W. Struve, who discovered the duplicity in
1827 when they were a mere 0.5 seconds of arc apart, referred to
them as the "closest of all neighbors." A current separation of
just 1.4 seconds (the two in slow orbit) still makes any kind of
measure of the individuals quite difficult, the "A" and "B" even
closer than the individual binaries of Epsilon Lyrae. And as a
result, very little is known about this lovely double. There being
no actual temperature measures, 9100 Kelvin is adopted from the
class. That and a distance of 342 light years (give or take 24)
give respective luminosities for Eps A and B of 80 and 55 Suns, with radii of 3.6 and 3.0 solar. A
measure of projected equatorial rotation speeds of 60 kilometers
per second for both (or for one or the other, it's hard to say)
give rotation periods under 3.0 and 2.5 days, fast enough with
unknown axial tilts to keep their atmospheres sufficiently stirred
to prevent peculiar abundances as a result of gravitational
settling and radiative lofting of various chemical elements.
Theory gives respective masses of 2.7 and 2.5 Suns and an age of
420 million years. Since the hydrogen-fusing lifetime of more-
massive Epsilon A is only 500 mega-years, it does not have long to
go before turning into a subgiant with a dead helium core and then
a true red giant, when it will
overwhelm Epsilon B.
The relative movement of the two gives an orbital period of 1216
years at an average separation of 228 Astronomical Units, an
orbital tilt of 84 degrees to the plane of the sky (which may well
not be shared by the stellar rotations), and an eccentricity that
runs the stars between 300 and 156 AU apart.
Kepler's laws then yield a mass-sum of 8.0 times that of the
Sun, too high, revealing quite-understandable orbital errors.
Adopting the evolutionary sum of 5.2 Suns and assuming the orbital
period to be correct (which it may well not be), the orbital size
shrinks to 196 AU. Out at a great distance of at least 15,000 AU
lies 13th magnitude Epsilon Ari C, which for a century has kept
close pace with its far brighter mates and may well belong to the
system. C's absolute brightness would make it a low-mass K7 dwarf
with an orbital period of at least 800,000 years, and we are back
to a system rather like Alpha and Proxima Centauri. At the nominal
values, from Epsilon C, the A-B team would appear as two Venus's
separated at most by around 4 minutes of arc. Most likely, though,
nobody is there to watch. (Thanks to Tim Benedictis for suggesting
this star and to David Bright for a Latin translation.)
Written by Jim Kaler 3/18/11. Return to STARS.