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.
Written by Jim Kaler. Return to STARS.