Using several instruments on ESO’s Very Large Telescope, VLT Survey Telescope and the New Technology Telescope, astronomers have discovered giant spots on the surface of extreme horizontal branch stars in three globular clusters: NGC 2808, NGC 6752 and NGC 5139.
An artist’s impression of what one of extreme horizontal branch stars, and its giant whitish spot, might look like. The spot is bright, takes up a quarter of the star’s surface and is caused by magnetic fields. As the star rotates, the spot on its surface comes and goes, causing visible changes in brightness. Image credit: ESO / L. Calçada, INAF-Padua / S. Zaggia.
Extreme horizontal branch (EHB) stars are objects with a mass of about 0.5 solar masses and temperatures in the range 20,000-30,000 K.
These stars are believed to have lost approximately 30% of their original mass and almost the entirety of their atmosphere.
Their hydrogen shell is so thin that, in the wake of exhausting their helium-burning core, they are doomed to skip the asymptotic giant branch phase; instead, they brighten and then become white dwarfs directly.
“These hot and small stars are special because we know they will bypass one of the final phases in the life of a typical star and will die prematurely,” said Dr. Yazan Momany, an astronomer at the INAF Astronomical Observatory of Padua.
“In our Galaxy, these peculiar hot objects are generally associated with the presence of a close companion star.”
“Surprisingly, however, the vast majority of these EHB stars, when observed in tightly packed stellar groups called globular clusters, do not appear to have companions.”
Dr. Momany and his colleagues observed EHB stars in NGC 2808, NGC 6752 and NGC 5139 globular clusters.
They found that many of these stars showed regular changes in their brightness over the course of just a few days to several weeks.
“After eliminating all other scenarios, there was only one remaining possibility to explain their observed brightness variations — these stars must be plagued by spots,” said Dr. Simone Zaggia, also from the INAF Astronomical Observatory of Padua.
The spots on EHB stars are brighter and hotter than the surrounding stellar surface, unlike on the Sun where astronomers see spots as dark stains on the solar surface that are cooler than their surroundings.
They are also significantly larger than sunspots, covering up to a quarter of the star’s surface.
They are incredibly persistent, lasting for decades, while individual sunspots are temporary, lasting only a few days to months. As the hot stars rotate, the spots on the surface come and go, causing the visible changes in brightness.
Beyond the variations in brightness due to spots, the researchers also discovered a couple of extreme horizontal branch stars that showed superflares.
“They are similar to the flares we see on our own Sun, but 10 million times more energetic,” said ESO astronomer Dr. Henri Boffin.
“Such behavior was certainly not expected and highlights the importance of magnetic fields in explaining the properties of these stars.”
The team’s paper was published in the journal Nature Astronomy.
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Y. Momany et al. A plague of magnetic spots among the hot stars of globular clusters. Nat Astron, published online June 1, 2020; doi: 10.1038/s41550-020-1113-4
