AK Scorpii

AK Scorpii is a young, actively feeding binary star system located roughly 459 light-years away in the constellation Scorpius. It is classified as a Herbig Ae/Be star and a spectroscopic binary, residing in the Upper Centaurus–Lupus star-forming region.

The two stars, each with a mass of 1.25 times that of the Sun, orbit one another every 13.6 days. Surrounding the pair is a narrow ring of dust about 30 astronomical units in radius, with gas filling the gap between the binary and the disk. This circumbinary disk has been directly imaged using the VLT/SPHERE instrument in scattered light and with ALMA.

The system is about 18 million years old. Its eccentric orbit causes variable rates of material falling onto the stars. At apastron, the farthest point in the orbit, gas is pulled from the inner edge of the disk into the gap, forming accretion streams that create ring-like structures around each star.

At periastron, the closest approach, these rings collide, causing a loss of angular momentum and triggering an accretion outburst. In August 2014, the Hubble Space Telescope observed a drop in hydrogen flux during periastron, attributed to a gas stream occulting one of the stars. XMM-Newton data also showed enhanced X-ray and ultraviolet flux at periastron, further indicating stronger accretion at that time.

The orbital plane of the binary and the inclination of the circumbinary disk are nearly aligned, meaning any circumbinary planet that forms would likely orbit in the same plane. The stars’ atmospheres are overabundant in yttrium, barium, and lanthanum; the primary star also shows excess zirconium, while the secondary is overabundant in sulfur.

The variable nature of AK Scorpii was first announced in 1907 by Henrietta Swan Leavitt and Edward Charles Pickering, and it received its variable star designation in 1912.