Alpha Octantis

Alpha Octantis is a star of magnitude 5.15 in the constellation Octans.

Orbit

Alpha Octantis is a spectroscopic binary system at a distance of 148 light years. It consists of two nearly identical stars that orbit in only 9.073 days, with a mean separation of 0.12 AU, and a high eccentricity that brings them between 0.08 AU and 0.17 AU apart. The pair has also been classified as a β Lyrae-type eclipsing binary system, but the calculated orbital tilt seems to be too high to allow eclipses.

Properties and Evolution

The stars are so close that they yield a blended, composite spectrum. The best guess is that they are F4 and F5 III giants, with a temperature of 6700 K. Their combined luminosity is 13.9 suns. The stars' luminosity and temperature imply individual masses of 2.0 suns each, and that they are not giants at all, but rather main-sequence stars with ages of 1.9 billion years - half of their expected 2.7-billion year hydrogen-fusing lifetimes.

The assumption that α Oct's components are identical gives them radii 2.0 times solar. A projected rotation speed of 71 km/sec gives them a rotation period of under two days. Their rapid rotation seems to promote magnetic activity, causing X-ray radiation. One or both components are "metallic line" stars, in which some metals (notably strontium) are enhanced by a diffusion process in which some elements are carried upward by radiation pressure, and others drift downward under gravity. A slight spectral variation of 2.9 days may be linked to actual rotation.

[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]