Using telescopes of the European Southern Observatory (ESO), astronomers discovered giant magnetic spots on the surface of very hot stars in star clusters. In addition to spots, some of these stars also have super-powerful flashes, explosive events, in which several million times more energy is released than with similar phenomena on the Sun. The study was published in the journal Nature Astronomy and is also reported in the ESO press release.
Scientists led by Yazan Momany of the Astronomical Observatory in Padua (INAF, Italy) investigated a special type of stars called stars of the extreme horizontal branch. These are objects with masses about half that of the Sun, but 4–5 times hotter than it. “These hot and small stars are distinguished by the fact that, as we know, they bypass one of the final stages of a typical star’s life and die prematurely,” Momani says. “In our Galaxy, these peculiar, unusual hot objects are usually part of a close binary system.”
However, it unexpectedly turned out that in the vast majority of stars of the extreme horizontal branch observed in densely populated star groups – globular clusters, no companion stars were noticeable. In addition, during long-term monitoring of such stars with ESO telescopes, the group discovered a new feature in these mysterious objects. Observing three globular clusters, Momani and his colleagues found regular changes in brightness in many stars of the extreme horizontal branch contained in them with periods of only a few days to several weeks. “After all the other scenarios were eliminated, there was only one opportunity to explain the observed brightness variability,” said Simone Zaggia, a researcher at the Astronomical Observatory in Padua (INAF, Italy) and a former ESO scholarship holder. “These stars must be stained!”
The spots on the stars of the extreme horizontal branch turn out to be completely different from the dark spots on our Sun, although in both cases they are caused by magnetic fields. The spots on these hot and extreme stars are brighter and hotter than the surrounding surface of the star, in contrast to the Sun, where the spots look like dark marks against a background of brighter and hotter surroundings. In addition, the spots on the stars of the extreme horizontal branch are much larger than the sun – they cover up to a quarter of the surface of the star. These spots are incredibly stable – they last for decades, and individual sunspots are short-lived formations, they last from several days to several months. When hot stars rotate, spots on the surface either appear in the field of view or disappear from it, which leads to the observed changes in brightness.
In addition to the brightness variability associated with the spots, the group discovered a pair of stars of the extreme horizontal branch on which superflashes occurred – explosive release of energy, which also indicates the presence of a magnetic field. “They look like flashes that we see on the sun, but are ten million times more powerful,” says Henry Boffin, an astronomer at ESO headquarters in Germany. – We did not expect such behavior in these stars; it shows how important it is to take magnetic fields into account when explaining their properties. ”
After sixty years of trying to understand the nature of the stars of the extreme horizontal branch, astronomers now have a more complete picture of their behavior. Moreover, this discovery can help explain the origin of strong magnetic fields in many white dwarfs – objects representing the last stage of the life of sun-like stars and having much in common with the stars of the extreme horizontal branch. “The main thing,” says team member David Jones, a former ESO scholar, and now an employee of the Canary Islands Institute of Astrophysics in Spain, “that the brightness changes of all the hot stars, from young stars like the Sun to old stars of the extreme horizontal branch and long dead white dwarfs can now be connected to each other. All these objects can have magnetic spots on the surface. ”
To reach this conclusion, astronomers used several ESO Very Large Telescope (VLT) receivers, including VIMOS, FLAMES and FORS2, as well as an OmegaCAM camera mounted on the VLT Survey Telescope at the Paranal Observatory. They also used the ULTRACAM receiver on the New Technology Telescope at the ESO La Silla Observatory, also in Chile. The breakthrough occurred when the group conducted observations of stars in the near ultraviolet region of the spectrum, which allowed astronomers to identify extremely hot stars among colder in globular clusters.