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ATIK (Omicron Persei). At the end of one of the star-streams thatmake most ofPerseus, just to the west ofbrightZeta Persei, lies seeminglymodest Atik, Omicron Persei, the name apparently referring to a"shoulder" of thePleiades, whichlies just to the south of the Hero's stars. Atik's fourthmagnitude (3.83, really not all that faint) status is the result ofconsiderable distance, direct measure giving 1475 light years (witha large uncertainty). The faintness is also caused by considerabledimming by interstellar dust in the Milky Way; a clear path wouldrender Atik about a magnitude brighter. Atik's chief attribute isits duplicity. It is aspectroscopicbinary (one detected through Doppler shifts in a compositespectrum) with a very short period of 4.419171 days in which a hotclass B (B1.5) giant with a temperature of 22,000 Kelvin mutuallyorbits another B star (this one a B3 dwarf 2.5 magnitudes fainterwith a temperature of 18,600). Mutual tides distort the components intoellipsoidal shapes, which makes the binary appear to vary over the orbital period by a few hundredths of a magnitude. The giant is evolvingwith a dead or near-dead helium core, while the dwarf is a commonhydrogen-fuser. The combined luminosity of the pair (withconsiderable allowance for ultraviolet light) comes in at 82,000solar, which given that the giant is 10 timesthe luminosity of the dwarf, leads to respective luminosities of75,000 and 7500 solar. Combination of data yield masses of 17 and8 solar. Each star rotates with an equatorial velocity greaterthan 85 kilometers per second. However, all is not well, sinceorbital analysis gives a bit of a different picture, withrespective luminosities of "only" 12,300 and 2000 solar, lowermasses of 10 and 7 solar, and radii of 7.6 and 4.0 solar (whichwith rotation speed gives rotation periods less than 4.5 and 2.4days). The problem is most likely one of distance. Atik lies atthe edge of the ability the Hipparcos satellite to produce goodparallaxes. Factoring in the errors could make the star as closeas 1000 light years, which would much narrow (but still not close)the gap between the two determinations. Atik is a source of X-raysthat suggest gasses at two temperatures. A hot gas at 3 millionKelvin is probably produced when the winds of the two starscollide, while a much hotter temperature of 16 million may be fromsome kind of hot corona (odd, since the stars should not have therequired magnetic fields). The giant is near the lower mass limitfor which stars explode, while the dwarf will become a massivewhite dwarf like Sirius B. Nearby, only a second of arc away, isan eight magnitude possible companion, about which nothing isknown. Atik's membership in the Perseus OB2 association of hot Oand B stars (which is associated with the cluster IC 348 and whichhouses both Zeta andXi Per) has longbeen disputed, the consensus now being that the star is not amember of the troup.
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