AK Scorpii

Lore & Background

In 1907 Henrietta Swan Leavitt and Edward Charles Pickering announced the discovery of this variable star, and it was given its variable star designation, AK Scorpii, in 1912. The binary consists of two stars with equal mass of 1.25 M☉, orbiting each other within 13.6 days. The binary is surrounded by a narrow ring of dust with a radius of about 30 astronomical units, and the gap between the binary and the dust disk is filled by some gas.

The binary is in an eccentric orbit that causes variable accretion rates due to gravitational interactions with the disk. During apastron, matter is dragged from the inner disk border into the gap, forming accretion streams that fill ring-like structures around each component. At periastron, the ring-like structures come into contact, leading to angular momentum loss and an accretion outburst. In August 2014 the system was observed with the Hubble Space Telescope during periastron passage, which observed a drop of hydrogen flux explained by occultation of the star by a gas stream. XMM-Newton data showed enhanced X-ray and ultraviolet flux during periastron, an additional sign of stronger accretion.

The orbital inclination of the binary and the inclination of the circumbinary disk are nearly the same, so any circumbinary planet that could form would orbit in the same plane. The spectrum of the stars shows their atmosphere is overabundant in yttrium, barium, and lanthanum; the primary additionally shows an overabundance of zirconium, and the secondary is overabundant in sulphur.

Reader's Guide

AK Scorpii is notable as a young, actively accreting binary star system that provides a natural laboratory for studying how binary stars interact with their surrounding circumbinary disk. The eccentric orbit of the binary causes periodic variations in accretion, with observable outbursts at periastron. The system's nearly coplanar binary and disk inclinations suggest that any circumbinary planets forming there would share the same orbital plane. Observations with the Hubble Space Telescope in August 2014 captured a drop in hydrogen flux during periastron, interpreted as occultation by a gas stream falling toward the star, while XMM-Newton data showed enhanced X-ray and ultraviolet flux, confirming stronger accretion at that orbital phase. The chemical overabundances of yttrium, barium, lanthanum, zirconium, and sulphur in the stellar atmospheres add further interest. The system's circumbinary disk has been imaged with VLT/SPHERE in scattered light and with ALMA, making it a key target for understanding disk evolution and planet formation around binary stars.

Did You Know?

• AK Scorpii is about 18 million years old.
• The binary stars each have a mass of 1.25 M☉ and orbit every 13.6 days.
• The circumbinary dust ring has a radius of about 30 astronomical units.