CHI LUP (Chi Lupi). Lying more or less between Scorpius and Centaurus, Lupus -- the Wolf -- is sometimes a bit
overlooked. Though second magnitude Kakkab (Alpha Lupi) leads the brightness
parade, it is not the best known of Lupus's stars, that honor
arguably going to mid-fourth-magnitude (3.95) Chi Lupi, which is
one oddball of a star, indeed TWO oddballs, since it is a close,
unresolved double. Chi Lupi is a "spectroscopic binary," one known to
be double from two overlapping spectra whose fingerprint
absorptions dance back and forth as a result of the Doppler effect, yielding an
orbital period of 15.2565... days. The brighter of the pair is a
class B (B9.5) subgiant with a temperature given at 10,650 Kelvin,
the fainter a cooler class A2 dwarf at T = 9200 Kelvin (though like
the class A star, the class B star is really a hydrogen-fusing
dwarf and not a true subgiant, which implies a more advanced
evolution). Their luminosities are difficult to separate, in
particular because each produces a different amount of ultraviolet
light. Roughly, the brighter class B star shines 90 or so times
more brightly than the Sun, the fainter roughly 25 times more,
luminosity and temperature yielding respective masses of 3 and 2.2
solar masses and a youthful age of less than 250 million years.
These masses, the orbital period, and the law that governs
gravitational interaction (Kepler's third law) indicate an orbital
separation of 0.21 Astronomical Units, about half the size of
Mercury's orbit about the Sun. None of these qualities make the
system very unusual. What stands out is that they are both
chemically peculiar, but in quite different ways. The fainter is
a "metallic-line star." Such stars, typically of classes A and F,
seem highly enriched in elements like copper, zirconium, barium,
and europium, at the same time being low in others such as calcium.
It is Chi Lupi's brighter member, however, that makes us notice.
It is a wonderful example of a "mercury-manganese" star (such stars
being typically of hotter class B). Its surface gases are enhanced
in platinum and gold by factors of tens of thousands, and it has a
million or so times the solar abundance of mercury. Again, calcium
is way down. Chi Lupi is so important that it has been designated
a "Hubble Pathfinder Star" that has been specially targeted by the
Space Telescope. These strange stars are not really rich in such
chemical elements, nor really depleted in others. Instead, they
have undergone a process of "diffusion," in which some elements
fall inward through the action of gravity, while others are lofted
upward and outward by radiation. Such separation can take place
only in the quiet atmosphere of a star that is not undergoing up-
and-down convection (which includes stars hotter than cool class A
or so) and that are rotating slowly, categories for which Chi Lupi
eminently qualify. (Chi Lupi is featured in Jim Kaler's "The Hundred Greatest Stars.")