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.
Written by Jim Kaler. Return to STARS.