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.")
Written by Jim Kaler. Return to STARS.