Serpentine spiral conceals massive binary star system

A swirl of dust and gas obscures a pair of massive stars orbiting each other, reports a paper published online this week in Nature Astronomy. Measurements of the velocities in the nebula suggest that at least one of the stars is rotating rapidly enough to potentially fire off a long-duration gamma-ray burst when it explodes as a supernova.

Cosmic pinwheels such as this one - which has been dubbed Apep, after the Ancient Egyptian serpent deity - are characteristic features of binary systems containing massive ‘Wolf-Rayet’ stars - those in the final evolutionary stage before going supernova. Each star in these systems ejects material outward in a ‘wind’, forming plumes of dust particles where the two winds collide. The relative motions of the dusty winds and the orbiting stars are reflected in the features of the gigantic enshrouding cloud, which can measure several light-years across.

Joseph Callingham and colleagues measured the speed of the gas in Apep’s nebula, finding it to be moving at an incredibly fast 3,400 km s-1. However, they found that the dust component is moving at a relatively sluggish 570 km s-1. This discrepancy in velocities is a product of some rapidly rotating stars - fast, hot winds are emitted from the star’s poles, while those at the equator are slower and denser.

Fast-rotating Wolf-Rayet stars are considered to be likely progenitors of long-duration gamma-ray bursts - the extremely energetic explosions released when massive stars collapse to form dense neutron stars or black holes. Apep thus provides both a unique opportunity to study near-critical rotation in such stars, and a rare, strong candidate for a gamma-ray burst source in our Galaxy.