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 F4F5 giants, but they've also been
called A7G2 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.