Brazilian scientists advance their understanding of the physics behind star superflares

Superflares release huge amounts of energy. If they occurred on our Sun, they would have a massive impact on Earth. The researchers studied two K-type stars and concluded that eruptions of this magnitude are due to their magnetic complexity.

By José Tadeu Arantes  |  Agência FAPESP – The relationship between sunspots and solar flares has been fairly thoroughly investigated in studies of the Sun, not least because these eruptions associated with ejections of coronal mass, in which massive amounts of energy are released, have a direct impact on our planet, causing more aurora borealis displays, radio communications blackouts, heightened effects of scintillation on GPS signals, and lower satellite velocities and altitudes (read more at: agencia.fapesp.br/41378).  

To explore the physics behind these stellar events in more depth, two Brazilian researchers set out to investigate superflares, intense phenomena that involve between 1,000 and 10,000 times more energy than the largest solar flares. In an analysis of two K-type stars, Kepler-411 and Kepler-210, they were surprised to discover that although the stars were similar in mass, rotation period and planetary system, and although both displayed about 100 starspots, the former produced 65 superflares and the latter none. An article on the study is published in Monthly Notices of the Royal Astronomical Society: Letters.

“Spot size doesn’t seem to be the main factor responsible for triggering superflares. An explanation might have to be sought in the magnetic complexity of the active regions,” said Alexandre Araújo, first author of the article. He is a teacher at the Integrated Center for Further and Adult Education (CIEJA) in the Campo Limpo district of São Paulo city (Brazil), and a postdoctoral fellow at Mackenzie Presbyterian University (UPM).

The study was supported by FAPESP, and conducted by Araújo and Adriana Valio, a researcher at UPM’s Center for Radio Astronomy and Astrophysics (CRAAM). She was thesis advisor for his PhD and is currently supervising his postdoc research.

The spots on both stars were characterized by measuring their intensity, temperature, position (latitude and longitude) and radius using a technique called planetary transit mapping. 

“The knowledge we gleaned from the literature suggested that stars with larger spots should be more likely to produce superflares, but that’s not what we saw. Kepler-411 had far smaller spots than Kepler-210 and theoretically should have had superflares, but none were observed. Our explanation for the absence of superflares on Kepler-210 despite the big spots is its magnetic complexity, and also the evolution and lifespans of the spots,” Araújo said.

Besides the pursuit of more detailed knowledge of stellar activities, the authors had an additional motivation for conducting the study. When superflares on solar-type stars were first discovered, the scientific community mainly wanted to find out whether such phenomena were possible on our Sun. Solar flares are far less intense and yet have a huge impact on life on Earth, so what should be expected of solar superflares, if there were any?

“Planets that orbit stars with frequent superflares can undoubtedly lose their atmosphere, making life impossible, or at least life as we know it,” Araújo said.

The structure of solar-type stars

To understand all this, let us pause for a moment and recapitulate some basic aspects of the structure of stars, known to scientists mostly from studies of the Sun. For clarity’s sake, we can think of this structure in terms of layers. “The core is the main source of a star’s energy. In the Sun, this region is a sphere with a radius that corresponds to a fifth of the Sun’s radius but with extremely high density. Thermonuclear reactions in the core convert hydrogen to helium, producing temperatures in the range of 13.6 million kelvin [K],” Valio explained.

The next layer is the radiative zone around the core, where energy is transported in all directions by photons – particles associated with electromagnetic radiation. Their propagation velocity in a vacuum is the highest in the material universe, but because the radiative zone is made up of particles (photons, electrons, etc.), absorption and emission by these components hinder the transit of photons, and they take around 1 million years to travel through this layer to the convection zone.

“In the convection zone, energy is transported by convection currents. The hottest matter rises to the surface of the star, while colder, denser matter sinks back down to the convective layer. This movement creates giant cells that transport energy and matter through the star. On the surface of the Sun, convection cells are called solar granules,” Valio said.

The Sun’s surface is known as the photosphere. This is where scientists observe sunspots, granules and flares throughout the chromosphere and corona, which together comprise the solar atmosphere. The average temperature in the photosphere is about 5,700 K, relatively cool compared with the inner layers of the Sun or the outer layers of its atmosphere. Most of the light and heat emitted by the Sun comes from the photosphere.

“The spots that appear in the photosphere are caused by intense magnetic fields and can last a few days or weeks. Their formation starts with a magnetic field generated by the movement of electrically charged particles in the tachocline, a thin layer between the radiative and convective zones. Magnetic flux tubes emerging on the Sun’s surface create intense fields that block the transfer of heat from the inner zones to the surface. The spots are dark because they are 1,000-1,500 degrees cooler than the rest of the surface,” Valio said.

Sunspots vary in size and shape. Their magnetic complexity is a key factor in the production of solar flares, which are observed throughout the electromagnetic spectrum, from radio waves to infrared, visible light, ultraviolet, X-rays and gamma rays. They are transient phenomena that occur in the solar atmosphere, in regions with high concentrations of magnetic field, where large amounts of energy are released by magnetic reconnection. The largest solar flares generate between 1,017 and 1,022 kilowatts (kW).

Planetary transit mapping

Discovering the mechanisms that create superflares is a major challenge for researchers. The association with starspots is widely accepted, but poorly understood. “Planetary transit mapping is an excellent method of investigating spots on the surface of solar-type stars,” Araujo said. “It’s currently the most robust method for this kind of investigation. However, it’s complicated to apply, mainly because of the difficulty of obtaining stars that meet the criteria for the investigation.”

He and Valio analyzed data from the Kepler space telescope in search of stars that matched the needs of the study. NASA designed Kepler to discover Earth-sized planets orbiting other stars outside the Solar System but still within the Milky Way galaxy (exoplanets). In the first part of its mission, from 2009 to 2013, it observed more than 150,000 stars by catching sight of tiny dips in the amount of light coming from individual stars caused by planets crossing in front of them. This is called the transit method.

Combing this immense database for objects that suited their purpose resembled looking for a needle in a haystack, Araújo noted. “First of all, the star had to have one or more planets,” he said. “For these exoplanets to be detected, their angle of inclination with respect to the star had to come within the telescope’s line of sight. In addition, the star had to have spots on its surface, and the exoplanet had to transit in the regions of the spots. Its orbital period should be only a few days long, and its radius should be larger than Earth’s so that the star’s light curves dimmed significantly. Last of all, the star had to have superflares.”

Fortunately, they succeeded in identifying a star, Kepler-411, that met all these criteria and, better still, had a planetary system with four exoplanets. However, to investigate the role of starspots, they needed to find a second star that was similar in every respect except one: it could not have superflares. “In a sense, it was overambitious of us to believe this second star existed. We were rewarded with detection of Kepler-210, which has stellar parameters that closely resemble those of Kepler-411,” Araújo said.

Many scientists believe the detection of superflares is directly linked to starspot temporal coverage, and the larger the area covered by spots, the larger the amount of magnetic energy stored up to produce superflares. “Our results offer a somewhat different perspective,” Araújo said. “In the case of Kepler-411, we detected 65 superflares with energy levels up to 1,035 ergs [1,035 × 10⁷ kilojoules]. Kepler-210 didn’t exhibit superflares even with double the temporal coverage, which made observation more probable. What surprised us most was that Kepler-411’s spot radii were far smaller than Kepler-210’s,” he added.

The explanation may be that although Kepler-210’s spots were larger, their magnetic configuration was simpler. “Sunspots are classified according to the behavior of the magnetic field in the area, as alpha (α), beta (β), gamma (γ) and delta (δ), or a combination of these. Delta sunspots produce more superflares. We believe the spots on Kepler-210 have a simpler magnetic configuration, similar to alpha or beta sunspots,” Valio said. “Unfortunately, accurate confirmation of this hypothesis would only be possible if we had magnetograms – images displaying the star’s surface magnetic field strength, location and intensity. Right now, these exist only for the Sun. We lack the technology to obtain magnetograms of distant stars. In any event, we can say on the basis of our study that it may be more productive to consider the magnetic complexity of the active regions, instead of focusing on the area of starspots.”

The article “The connection between starspots and superflares: a case study of two stars” is at: academic.oup.com/mnrasl/article-abstract/522/1/L16/7079139?redirectedFrom=fulltext.