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10th of February 2021
An extreme collision of stellar winds
by Marcote et al.
Apep is a binary star system, named after the Egyptian god of chaos due to the spiral pattern of dust generated by its two member stars. Marcote et al. have looked at Apep’s heart with the highest angular resolution available and revealed the strong shock produced by the collision of the extreme stellar winds. Apep is comprised of two Wolf-Rayet stars, which represent the very last stages in the life of the most massive stars before they collapse to produce a supernova explosion. Apep is unusual as it is the by far brightest of such colliding-wind systems, especially at radio wavelengths.

To understand the origin of this surprising brightness, Marcote et al. conducted radio observations of Apep with the Southern Hemisphere Long Baseline Array (LBA). The observations combined data from ten radio telescopes spread across Australia and New Zealand, and achieved and angular resolution that is sharp enough to identify a truck on the surface of the Moon when looking from the Earth. This angular resolution was sufficient to reveal the origin of the radio emission -- an extreme shock produced by the collision of the two stellar winds. The two Wolf-Rayet stars in Apep total 20 times the mass of the Sun, and exhibit winds of up to millions of kilometers per hour. When these two winds collide they produce a very strong shock, which is observed as a banana-shaped structure between the two stars. The results are presented in a paper published in Monthly Notices of the Royal Astronomical Society. The image above shows images of the dust spiral seen in Apep at infrared wavelengths and, right at its center, the region where the two stellar winds collide and emitting in radio. The inset shows the radio emission as the blue structure in the inset, where the two stars represent their real positions as determined from optical observations. (Image credits: B. Marcote & ESO/Callingham)

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