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KAPPA CNC (Kappa Cancri).Cancer, thecelestial Crab, is best known for an irregular four-star basketthat holds theBeehive (or Praesepe)cluster. One pays little attention to the outlying stars that makethe crablegs, one of which is fourth rankingAcubens, Alpha Cancri. We pay even lessheed to the rest of the gang, including obscure, fifth magnitude(5.29, approaching sixth)Kappa Cancri. A closer look, however, reveals a fascinating, though stillunderstudied,double (andpossibly triple) star. The dominant one is a white class B (B8)giant that is coupled with what appears to be a mid class A dwarf. In the sometimes confusing lingo of astronomy, the B star is calledKappa Cnc A (as it is the brighter), the A star Kappa Cnc B. (Holdon to that thought.) Only 0.3 seconds of arc apart at lastmeasure, the two can be separated only by sophisticatedinstrumentation. The closeness makes magnitude determinationtricky. Several values are noted; the best (from the observedlight ratio of 11.5) brings the much brighter class B giant (KappaA) in at magnitude 5.33, the class A dwarf appearing at a dim 7.98. From its distance of 484 light years and a temperature of 13,470 K,Kappa A's luminosity comes in at 315 times that of theSun, which in turn gives a radius of 3.3 timessolar, and a mass of 4.0 solar. The star is also then seen not tobe agiant at all, but a dwarfabout halfway through its 165 million year hydrogen-fusinglifetime. The fainter star's respective values are 8500 Kelvin, 11solar luminosities, 1.6 solar radii, and 1.9 solar masses (showinghow dependent luminosity is on mass and evolution). The projectedseparation of 42 Astronomical Units leads to a period of at least114 years. Lunar occultation measures suggest a third star muchcloser to the B giant with a 6.9 day period, giving a separation ofonly a bit over a tenth of an AU. The fascination of the systemlies in the chemical composition. Kappa A is a "mercury-manganesestar" in the mold ofChi Lupi, in whichmanganese is elevated (relative to solar proportions) by a factorof 170 and mercury by an astounding 43,000! And there's gold inthem thar hills, the precious element elevated some 15,000 times. At the same time, other elements like magnesium are depleted. Theorigin of such seeming nonsense is diffusion, in which stellarradiation lofts some elements upward, while gravity causes othersto sink. This particular version of the phenomenon is seen in therealm of the cooler B stars. Not to be outdone, the companion maybe a "metallic-line" star, in which a different set of elements isaffected. We see the phenomena only in slowly rotating stars, inwhich the gases are relatively undisturbed and unmixed, andconsistently Kappa A spins at just 7 kilometers per second(projected), though Kappa B is a bit faster at 40. All in all,Kappa is not a bad catch for a seemingly obscure star, one thattakes a dark night to find. Written byJim Kaler 5/11/07. Return toSTARS.
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