[no subject]

STAR OF THE WEEK: ALPHA OCT (Alpha Octantis).  Alpha stars, like
"alpha anything," are supposed to dominate their groups.  Here's
one that quite violates "Bayer's rule," that "Alpha" belongs to the
brightest star in a constellation.  Of course Bayer could not see,
and did not name, the faint stars of the modern constellation
Octans, the Octant, which surrounds the South Celestial Pole, and
in which fifth magnitude (5.15) Alpha Octantis is rather far down
the list, Nu Oct being the brightest of them (which is not saying
all that much).  Yet Alpha Octantis still has a few things to
recommend it, one of which is mystery, the star not only lacking in
brightness but in our knowledge of its properties.  It IS double,
it MAY be eclipsing though other observations say it CAN'T be
eclipsing, the stars SEEM to be giants, yet they are NOT.  Even the
spectral and luminosity classes are controversial.  At least the
distance of 148 light years seems well known.  Alpha Octantis is a
spectroscopic binary that consists of two nearly identical stars
that are so close that they orbit in only 9.073 days, and yield a
blended, composite spectrum.  The best guess is that they are F4+F5
giants, but they've also been called A7+G2 giants, quite a
divergence.  We'll stick here with class F and with a singular
temperature measure of 6700 Kelvin.  Their combined luminosity is
13.9 times that of the Sun.  The assumption that they are identical
obviously gives just 7 solar for each, radii of 2.0 solar (pretty
small for "giants"), and from a single projected rotation speed of
71 kilometers per second, a rotation period of under two days.  The
quick rotation seems to promote magnetic activity, explaining a
rather high rate of X-ray radiation, though which star is
responsible is not known.  One or both is/are classed as "metallic
line," in which some metals (notably strontium) are enhanced by
diffusion, some elements moving up from radiation pressure, others
drifting down under gravity.  A slight variation of 2.9 days may be
linked to actual rotation (the above spin-speed not well known). 
Luminosity and temperature tell of stars that each carry twice the
mass of the Sun, but reveal that the twin Alpha Oct members are not
giants at all, but hydrogen-fusing dwarfs with ages of 1.9 billion
years, rather far from their 2.7 billion year expected lifetimes. 
The masses and short orbital period give a mean separation of 0.12
Astronomical Units, just 26 solar radii.  Close indeed!  Spectral
observations tell of a high eccentricity that runs them between
0.17 and 0.08 AU apart, the stars young enough that the mutual
orbits have not yet become circular.  The pair is also classed as
an eclipser, but the calculated orbital tilt seems to be too high
to allow eclipses, sticking us with yet another mystery from one of
the most neglected Alpha stars of the sky.



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Jim Kaler
Professor Emeritus of Astronomy   Phone: (217) 333-9382
University of Illinois            Fax: (217) 244-7638
Department of Astronomy           email: kaler@xxxxxxxxxxxxxx
103 Astronomy Bldg.               web: http://www.astro.uiuc.edu/~kaler/
1002 West Green St.
Urbana, IL 61801
USA

Visit: http://www.astro.uiuc.edu/~kaler/ for links to:
   Skylights (Weekly Sky News updated each Friday)
     Stars (Portraits of Stars and the Constellations)
       The StarGazer (a new planetarium show)
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