ATIK (Omicron Persei). At the end of one of the star-streams that
make most of Perseus, just to the west of
bright Zeta Persei, lies seemingly
modest Atik, Omicron Persei, the name apparently referring to a
"shoulder" of the Pleiades, which
lies just to the south of the Hero's stars. Atik's fourth
magnitude (3.83, really not all that faint) status is the result of
considerable distance, direct measure giving 1475 light years (with
a large uncertainty). The faintness is also caused by considerable
dimming by interstellar dust in the Milky Way; a clear path would
render Atik about a magnitude brighter. Atik's chief attribute is
its duplicity. It is a spectroscopic
binary (one detected through Doppler shifts in a composite
spectrum) with a very short period of 4.419171 days in which a hot
class B (B1.5) giant with a temperature of 22,000 Kelvin mutually
orbits another B star (this one a B3 dwarf 2.5 magnitudes fainter
with a temperature of 18,600). Mutual tides distort the components into
ellipsoidal shapes, which makes the binary appear to vary over the
orbital period by a few hundredths of a magnitude.
The giant is evolving
with a dead or near-dead helium core, while the dwarf is a common
hydrogen-fuser. The combined luminosity of the pair (with
considerable allowance for ultraviolet light) comes in at 82,000 solar, which given that the giant is 10 times
the luminosity of the dwarf, leads to respective luminosities of
75,000 and 7500 solar. Combination of data yield masses of 17 and
8 solar. Each star rotates with an equatorial velocity greater
than 85 kilometers per second. However, all is not well, since
orbital analysis gives a bit of a different picture, with
respective luminosities of "only" 12,300 and 2000 solar, lower
masses of 10 and 7 solar, and radii of 7.6 and 4.0 solar (which
with rotation speed gives rotation periods less than 4.5 and 2.4
days). The problem is most likely one of distance. Atik lies at
the edge of the ability the Hipparcos satellite to produce good
parallaxes. Factoring in the errors could make the star as close
as 1000 light years, which would much narrow (but still not close)
the gap between the two determinations. Atik is a source of X-rays
that suggest gasses at two temperatures. A hot gas at 3 million
Kelvin is probably produced when the winds of the two stars
collide, while a much hotter temperature of 16 million may be from
some kind of hot corona (odd, since the stars should not have the
required magnetic fields). The giant is near the lower mass limit
for which stars explode, while the dwarf will become a massive
white dwarf like Sirius B. Nearby, only a second of arc away, is
an eight magnitude possible companion, about which nothing is
known. Atik's membership in the Perseus OB2 association of hot O
and B stars (which is associated with the cluster IC 348 and which
houses both Zeta and Xi Per) has long
been disputed, the consensus now being that the star is not a
member of the troup.