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.