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.