KAPPA CNC (Kappa Cancri). Cancer, the celestial Crab, is best known for an irregular four-star basket that holds the Beehive (or Praesepe) cluster. One pays little attention to the outlying stars that make the crablegs, one of which is fourth ranking Acubens, Alpha Cancri. We pay even less heed to the rest of the gang, including obscure, fifth magnitude (5.29, approaching sixth) Kappa Cancri. A closer look, however, reveals a fascinating, though still understudied, double (and possibly 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 called Kappa Cnc A (as it is the brighter), the A star Kappa Cnc B. (Hold on to that thought.) Only 0.3 seconds of arc apart at last measure, the two can be separated only by sophisticated instrumentation. The closeness makes magnitude determination tricky. Several values are noted; the best (from the observed light ratio of 11.5) brings the much brighter class B giant (Kappa A) 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 the Sun, which in turn gives a radius of 3.3 times solar, and a mass of 4.0 solar. The star is also then seen not to be a giant at all, but a dwarf about halfway through its 165 million year hydrogen-fusing lifetime. The fainter star's respective values are 8500 Kelvin, 11 solar luminosities, 1.6 solar radii, and 1.9 solar masses (showing how dependent luminosity is on mass and evolution). The projected separation of 42 Astronomical Units leads to a period of at least 114 years. Lunar occultation measures suggest a third star much closer to the B giant with a 6.9 day period, giving a separation of only a bit over a tenth of an AU. The fascination of the system lies in the chemical composition. Kappa A is a "mercury-manganese star" in the mold of Chi Lupi, in which manganese is elevated (relative to solar proportions) by a factor of 170 and mercury by an astounding 43,000! And there's gold in them thar hills, the precious element elevated some 15,000 times. At the same time, other elements like magnesium are depleted. The origin of such seeming nonsense is diffusion, in which stellar radiation lofts some elements upward, while gravity causes others to sink. This particular version of the phenomenon is seen in the realm of the cooler B stars. Not to be outdone, the companion may be a "metallic-line" star, in which a different set of elements is affected. We see the phenomena only in slowly rotating stars, in which the gases are relatively undisturbed and unmixed, and consistently 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 that takes a dark night to find.
Written by Jim Kaler 5/11/07. Return to STARS.