EPS CAP (Epsilon Capricorni). A couple degrees southwest of brighter
Gamma and Delta Capricorni (of Capricornus, the Zodiac's Water Goat) lies more or
less obscure Epsilon Capricorni, which was clearly given the fifth
letter of the Greek alphabet by Bayer because
of its proximity to Gamma and Delta rather than for its dim fifth
magnitude (4.68) brightness. The star is a surprising gem. It's a
hot class B (B2.5) emission line dwarf (a "Be" star similar to Gamma Cassiopeiae and Delta Scorpii) with a surrounding disk and
also a close double. Five degrees south of the ecliptic, Eps Cap can be occulted
by the Moon, which reveals the presence of two stars, the brighter (Eps Cap
Aa) at magnitude 5.0, the other (Eps Cap Ab) at 6.3. At a substantial
distance of 1055 light years (plus or minus 62) and not that far off
the Milky Way in Sagittarius to the west, Epsilon Cap is dimmed
about 0.2 magnitudes by interstellar
dust. The temperature of the brighter, Eps Cap Aa, is measured
at 19,780 Kelvin, just right for a B2.5 dwarf. From its absolute
visual brightness, the fainter should be a B4 dwarf with a temperature
of 17,000 Kelvin. After correction for a lot of ultraviolet light, we find the
respective luminosities to be 5600 and 1235 times that of the Sun, which give the stars radii of 6.4 and 4.1
times solar. Theory then yields masses of 8.5 and 5.8 Suns. The angular
separation found from the occultation yields a physical distance
between the two of 1.5 Astronomical Units, about the distance of Mars
from the Sun, which because of foreshortening could be a serious
underestimate. Kepler's laws
then give an orbital period of at least half a year.
The separation between Aa and Ab is sufficient to allow a rotating disk
around the brighter star that is somehow formed by the rapid observed
rotation speed of 218 kilometers per second, which gives Eps Aa a
rotation period of under 1.48 days. Radiating emission lines of hydrogen, the
disk varies in brightness with a period of 0.99 days. If the disk and
star are rotating with the same period (which seems unlikely), the
stellar rotation axis would be tilted by 42 degrees to the line of
sight, giving the star an upper limit to the true rotation speed of
325 km/s. Given the high rotation velocity, which mixes the stellar
gases, it's rather odd that Epsilon Capricorni is a "silicon star" in
which silicon's abundance is raised by elemental separation. Not far
from its hydrogen-fusing lifetime of 30 million years, the brighter
of the close pair is near the limit at which it could blow up as a supernova, perhaps ejecting
the fainter "Ab" as a runaway star. If it doesn't, Eps Cap Aa will
evolve into a massive white
dwarf. If "Ab" is drawn close enough by evolutionary processes
to the dying "Aa, the latter could exceed the white dwarf limit and
still blow up as a classic "Type
Ia" supernova. Off in the distance, at separations of 66 and 63
seconds of arc are tenth and fourteenth magnitude companions, Eps Cap
B and C, which are moving far too fast relative to far brighter Eps
Cap proper to be gravitationally bound, and are thus just line of sight
coincidences.
Written byJim Kaler 10/09/15. Return to STARS.