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Extreme collision of stellar winds between two massive stars
Intensive observations carried out by an international team of scientists - including Michaël De Becker, astrophysicist at the ULiège - explain the origin of the powerful radio wave emission of two of the three very massive stars of the Apep star system. These observations have made it possible to produce images, with a level of detail never before achieved, of a system such as there must be only a few in our galaxy.
he star system known as Apep, God of the Egyptian Pantheon, whose name means "Giant Serpent", located some 8000 light years from our Earth, was recently discovered thanks to its spectacular spiral structure of dust. This remarkable structure comes from two Wolf-Rayet stars located at its heart. Wolf-Rayet stars are hyper-massive stars that have entered the last stage of their lives, when they will be stripping their outer layers significantly and quite rapidly. This expulsion of matter results in what are called stellar winds, winds that carry not only a large amount of matter, but above all an enormous amount of energy. What is very interesting and almost unique here is that we are facing two massive stars orbiting each other, both in extreme phases of their evolution before the supernova explosion," explains Michaël De Becker, astrophysicist at the ORCA Laboratory (STAR Institute / Faculty of Sciences) of the University of Liège, which participated in the study. The collision of their stellar winds also gives rise to a substantial acceleration of particles revealed by a particular radio wave emission. »
In order to understand the origin of this strong luminosity and to visualise what is happening at the heart of this spiral of dust, an international group of astronomers has carried out radio observations of the system using the Australian Long Baseline Array (LBA), an array of telescopes that combines data from ten radio telescopes spread across Australia and New Zealand, with a resolution fine enough to identify a truck on the Moon's surface from Earth. By pointing their telescopes in the direction of Apep, the team of scientists revealed details about the origin of this strong radio emission: the light comes specifically from the extreme shocks produced by the collision of the two stellar winds.
"The two Wolf-Rayet stars at the heart of Apep have very powerful stellar winds with velocities up to millions of kilometres per hour," explains Benito Marcote from the Joint Institute for VLBI ERIC (JIVE) in the Netherlands who led the study. When these two winds collide, they produce extreme shocks, never before observed with such an intensity in our Galaxy. These shocks are observed in the form of a banana-shaped structure that emits high intensity radio waves. "According to Michaël De Becker, this phenomenon can be explained by synchrotron radiation. "The energy deployed in the wind collision is such that the intensity of this radio emission is unprecedented for this type of system made up of massive stars. Upstream of this radiation, we are dealing with the activity of a real particle accelerator fed by the power of the stellar winds and responsible for the high-energy electrons that generate this radiation. »
Thanks to Apep's observations, researchers are beginning to understand how these very massive stars can behave, the dynamics of their strong stellar winds, and how huge amounts of dust are produced after their winds collide. This dust, which is expelled from the system by following the orbital motion of the two stars, is responsible for the beautiful spiral structure revealed in infrared light.
Although these stars are much more massive than our Sun, their lifetime is much shorter. Unlike our Sun, which will end its life by seeing its core collapse and become a white dwarf," Michaël De Becker continues, "Wolf-Rayet stars end their life as a supernova, an explosion that generates an tremendous amount of energy." If the environment around the system is already much more extreme than any other known system involving two massive stars, astronomers expect the system to end its life in an extreme explosion. Extreme enough to make Apep the most likely system to produce a gamma-ray burst - a very energetic flash of gamma rays that to date has only been observed at cosmological distances, never in our Galaxy.
Marcote, Callingham, De Becker, et al., AU-scale radio imaging of the wind collision region in the brightest and most luminous non-thermal colliding wind binary Apep, 2021, MNRAS, 501, 2, 2478.