Stars haunted by huge magnetic spots

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01 June 2020, 11:26 p.m.

With the help of telescopes from the European Southern Observatory (ESO), astronomers have discovered huge spots on the surface of extremely hot stars that are hiding in star clusters. Not only are these stars plagued by magnetic spots, some also experience superflare events, high-energy explosions that are several million times stronger than similar eruptions on the sun. A press release from the ESO Science Outreach Network (ESON).


Enlarge pictureWith the help of ESO telescopes, astronomers have discovered huge spots on the surface of extremely hot stars that hide in star clusters, which are called extreme horizontal branch stars. This picture is an artistic representation of the impression of one of these stars and its huge, flecky spot. The spot is bright, occupies a quarter of the star’s surface, and is caused by magnetic fields. As the star rotates, the spot appears and disappears on its surface, causing visible changes in brightness.
(Image: ESO / L. Calada, INAF-Padua / S. Zaggia)

The results published today in Nature Astronomy help astronomers better understand these mysterious stars. They open doors to clarify other difficult to solve mysteries of star astronomy.

The team, led by Yazan Momany from the INAF Astronomical Observatory in Padua, Italy, investigated a special type of star known as extreme horizontal asterisk objects about half the mass of the sun but four to five times hotter. These hot and small stars are special because we know they will bypass one of the last stages of a typical star’s life and will die prematurely, says Momany, who previously worked as an astronomer at ESO’s Paranal Observatory in Chile. In our galaxy, these strange hot objects are generally associated with a nearby companion star.

Surprisingly, however, the vast majority of these extreme horizontal branch stars, when observed in densely packed star groups called globular clusters, appear to have no companions. The long-term observation of these stars by the research group, which was carried out with ESO telescopes, also showed that there was something more to these mysterious objects. When examining three different globular clusters, Momany and his colleagues found that many of the extreme horizontal branch stars contained therein showed regular changes in brightness over the course of just a few days to several weeks.

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After all other scenarios had been eliminated, there was only one way to explain the observed fluctuations in brightness, concludes Simone Zaggia, a co-author of the study by the INAF Astronomical Observatory in Padua in Italy and a former ESO scholarship holder: these stars have spots be covered!

Spots on extreme horizontal stars seem to be very different from the dark sun spots on the sun. But both are caused by magnetic fields. The spots on these hot, extreme stars are lighter and hotter than the surrounding star surface, in contrast to the sun, where we see spots as dark discolourations on the sun surface that are cooler than their surroundings. The spots on extreme horizontal star stars are also significantly larger than sun spots and cover up to a quarter of the star surface. These spots are incredibly persistent and last for decades, while individual sun spots are temporary and only last for a few days to months. As the hot stars rotate, the spots appear and disappear on the surface and cause the visible changes in brightness.

In addition to the changes in brightness due to the spots, the team also discovered some extreme horizontal stars, the superflares showed sudden energy outbursts and another sign of the existence of a magnetic field. They resemble the flares that we see on our sun, but they are ten million times more energetic, says co-author of the study, Henri Boffin, an astronomer at ESO’s German headquarters. Such behavior was certainly not to be expected and underlines the importance of magnetic fields in explaining the properties of these stars, says study co-author Henri Boffin.

After trying to understand extreme horizontal branch stars for six decades, astronomers now have a more complete picture of them. In addition, this result could help explain the origin of strong magnetic fields in many white dwarfs, objects that represent the last stage in the life of sun-like stars and are similar to extreme horizontal stars. The overriding importance, according to team member David Jones, a former ESO scholarship holder who is now working at the Instituto de Astrofsica de Canarias, Spain, is that the changes in the brightness of all hot stars range from young, sun-like stars to old, extreme horizontal branches White dwarfs who have long since died could all be connected. These objects can therefore be understood as being collectively affected by magnetic spots on their surface.

ESO / L. Cal

Enlarge pictureSpots on extreme horizontal star stars (right) appear to be very different from the dark sun spots on the sun (left). However, both are caused by magnetic fields. The spots on these hot, extreme stars are lighter and hotter than the surrounding star surface, in contrast to the sun, where we see spots as dark discolourations on the sun surface that are cooler than their surroundings. The spots on extreme horizontal star stars are also significantly larger than sun spots and cover up to a quarter of the star surface. While the sunspots vary in size, a typical size corresponds to an earth-sized planet, 3000 times smaller than a huge spot on an extreme horizontal star.
(Image: ESO / L. Calada, INAF-Padua / S. Zaggia)

To achieve this result, the astronomers used several instruments from ESO’s Very Large Telescope (VLT), including VIMOS, FLAMES and FORS2, and OmegaCAM, which is attached to the VLT Survey Telescope (VST) of the Paranal Observatory. They also use ULTRACAM on the New Technology Telescope (NTT) at ESO’s La Silla Observatory, also in Chile. The breakthrough came when the team observed the stars in the near ultraviolet part of the spectrum, which allowed them to see the hot, extreme stars that are brightly protruding from the colder stars in globular clusters.

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additional Information
This study is presented in the work “A plage of magnetic spots among the hot stars of globular clusters” published today in Nature Astronomy (doi: 10.1038 / s41550-020-1113-4).

The team consists of Y. Momany (INAF Padua Astronomical Observatory, Italy [INAF Padua]), S. Zaggia (INAF Padua), M. Montalto (Department of Physics and Astronomy, University of Padua, Italy [U. Padua]), D. Jones (Instituto de Astrofsica de Canarias and Department of Astrophysics, University of La Laguna, Tenerife, Spain), HMJ Boffin (European Southern Observatory, Garching, Germany), S. Cassisi (INAF-Astronomical Observatory of Abruzzo and INFN Pisa, Italy), C. Moni Bidin (Instituto de Astronomia, Universidad Catolica del Norte, Antofagasta, Chile), M. Gullieuszik (INAF Padua), I. Saviane (European Southern Observatory, Santiago, Chile), L. Monaco (Departamento de Ciencias Fisicas , Universidad Andreas Bello, Santiago, Chile), E. Mason (INAF Astronomical Observatory of Trieste, Italy), L. Girardi (INAF Padua), V. D’Orazi (INAF Padua), G. Piotto (U. Padua), AP Milone (U. Padua), H. Lala (U. Padua), P.B. Stetson (Herzberg Astronomy and Astrophysics, National Research Council, Victoria, Canada) and Y. Beletsky (Las Campanas Observatory, Carnegie Institution of Washington, La Serena, Chile).

about the ESO
The European Southern Observatory ESO) is the leading European organization for astronomical research and the most scientifically productive observatory in the world. The organization has 16 member countries: Belgium, Denmark, Germany, Finland, France, Great Britain, Ireland, Italy, the Netherlands, Austria, Poland, Portugal, Spain, Sweden, Switzerland and the Czech Republic. In addition, the host country Chile and Australia are strategic partners. ESO is implementing an ambitious program that focuses on the design, construction, and operation of high-performance, ground-based observatories that enable astronomers to make important scientific discoveries. The organization also plays a major role in promoting international cooperation in the field of astronomy.

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ESO has three unique observation locations in Chile: La Silla, Paranal and Chajnantor. On the Paranal, ESO operates the Very Large Telescope (VLT) and the world‘s leading Very Large Telescope Interferometer, as well as two surveying telescopes: VISTA in the infrared range and the VLT Survey Telescope (VST) for visible light. In the future, ESO will also host and operate the Cherenkov Telescope Array South, the largest and most sensitive gamma-ray observatory in the world. ESO is also one of the main partners in two projects on Chajnantor, APEX and ALMA, the largest astronomical project at all. On the Cerro Armazones near the Paranal, ESO is currently building the Extremely Large Telescope (ELT) with a diameter of 39 meters, which will one day be the largest optical telescope in the world.

The translations of English-language ESO press releases are a service of the ESO Science Outreach Network (ESON), an international astronomical public network in which scientists and science communicators from all ESO member countries (and some other countries) are represented. This is the German node of the network House of astronomy in Heidelberg.

Source: ESO ESON

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