IOTA LUP (Iota Lupi). The southern constellations of Centaurus, Scorpius, Lupus, and Crux are filled with hot blue stars of classes O and particularly B, massive stars recently born and arranged in huge expanding associations that are not bound together by gravity but that still imply common birthplaces and times. Though not assigned to any of the known systems, Iota Lupi, a hot (19,000 Kelvin) class B (B2.5) "subgiant" (but see below), still fits in with all the other hot stars. At the bright end of fourth magnitude (3.55, nearly third), Iota is among the brighter neglected stars, the result of its position 46 degrees south of the celestial equator. The westernmost Greek lettered star in Lupus (the Wolf), just off the border with Centaurus, it possibly belongs to the "Upper Centaurus Lupus" association, but at a distance of 338 light years seems too close (UCL centered at 450 light years). More important, the relative motions are not consistent with membership. From distance, temperature (needed to account for a rather large amount of invisible ultraviolet light), and a correction for 0.22 magnitudes of dimmimg by interstellar dust, the star radiates at a total rate of 2050 Suns, from which we find a radius of 4.2 times solar, all rather similar to Theta Lupi, which IS a member of UCL. Like many of its kind, Iota Lup is a fast rotator, but how fast is open to some question, the projected equatorial speeds ranging from 222 kilometers per second to a huge 370. The slower value, which gives a rotation period of about 0.9 days, is newer and probably closer to the mark. Whatever the rotation, there is no evidence of a surrounding disk that would make Iota a "B-emission" star like Gamma Cassiopeiae, Zeta Tauri, or Delta Scorpii. Given the temperature and luminosity, the theory of stellar structure and evolution gives a hefty mass of 6.5 times that of the Sun, and shows that the star is not a subgiant in the evolutionary sense but a dwarf roughly midway through its hydrogen fusing lifetime of 60 or so million years. (Such discrepancies are common, especially among the hotter stars. Just because a star is classified spectrally as a subgiant does not necessarily imply that it is in the subgiant evolutionary state, wherein hydrogen fusion has ceased or will do so shortly.) Not massive enough to blow up as a supernova, Iota Lupi's fate is to die as a white dwarf of about a solar mass similar to the companion to Sirius in Canis Major.

Written by Jim Kaler 8/09/13. Return to STARS.