DELTA LIB (Delta Librae). Not really given its due, fifth magnitude Delta Librae (4.92, in Libra, the Scales) is a classic interacting eclipsing binary that has been called "The Algol of the South." Beyond that the star is a bit of a mess. It is sometimes referred to as "Zubenhakrabi" (the Scorpion's Claw), though that name was actually assigned to Gamma Scorpii by Bayer (and erroneously by nineteenth century Elijah Burritt to Eta Librae). But Gamma Sco seemed more to belong to Libra, hence was given the name Sigma Lib (where it seems best to lie today), leaving Delta Lib with no classical proper name at all. Except that of the Southern Algol. Most of the light comes from a class B (B9.5 or sometimes A0) dwarf, which has a class K (K0) subgiant companion. They undergo a primary eclipse, wherein the fainter evolved K0 star gets partially in front of the bright B9.5 star every 2.327 days, the magnitude dropping from 4.9 to 5.9, a factor of 2.5. Velocity and eclipse data give an orbital radius of just 14 solar radii, 0.064 Astronomical Units, nearly identical radii for the two stars of 4.2 times that of the Sun, and respective masses for the hot and cool components of 4.7 and 1.8 solar. The odd thing about the system, and about all "Algols," is that the more evolved of the two, in this case the subgiant, is the less massive, theory saying that the MORE massive star of a pair should evolve first. What happens is that when the more massive expands to become a giant, it overflows the zero-gravity tidal surface imposed by the companion, which sends mass to the hotter dwarf, thus reducing the evolved star's mass to far less than what it started with. One star is literally destroying the other. From a temperature of the B dwarf of 10,100 Kelvin and a distance of 294 light years (give or take 16, orbital data giving 323), the B star (ignoring the companion) has a luminosity of 89 Suns and a radius of 3.1 solar, less than that given above. Theory then reveals a mass of 2.9 times that of the Sun, in rather wild disagreement with that found from the orbit, the star then becoming quite the anomaly. An equatorial rotation velocity of 79 kilometers per second gives a rotation period of under 2.9 days, consistent with the orbital period. From a century-old paper, the star has the distinction of being the first for which for which it was suggested that orbital spectroscopic data might yield a rotation velocity. All the orbital data also suggest a rather mysterious third solar type star that orbits the inner pair at a distance of about 4 AU. (These two companions to the B9.5 star should not be confused with faint Delta Lib B and C that lie 68 and 103 seconds of arc away. "B" is just in the same line of sight, while "C," which may itself be a double K-M dwarf, is problematic.) Delta Lib, the southern Algol, is thus ripe for further studies to iron it all out, such examinations critical to how stars live and die. (Thanks to John Say, who suggested this star. Orbital results are from a paper by V. Bakis et al. that appeared in the Monthly Notices of the Royal Astronomical Society, vol. 370, p. 1935, 2006.)

Written by Jim Kaler 9/28/12. Return to STARS.