Brazilians advance in understanding the physics behind stellar superexplosions – 06/23/2023 – Science

The relationship between sunspots and solar flares has been extensively investigated in studies of the Sun. Especially because these eruptions associated with coronal mass ejections, in which large amounts of energy are released, directly impact our planet, causing a greater occurrence of aurora borealis; blackouts in radio communications; increased flicker effect on GPS signals; reduction in speeds and altitudes of artificial satellites (read more here).

To understand the physics behind these stellar events, new research has focused on an even more intense phenomenon called a superflare, with energy 1,000 to 10,000 times greater than the largest explosions seen on the Sun. And he looked for this type of event in two K-type stars: Kepler-411 and Kepler-210. He discovered —to the researchers’ surprise— that despite the fact that these stars were similar in every way, from masses to rotational periods and planetary systems, and that both exhibited around 100 spots, the first one produced 65 superflares, while the second produced none. An article about it was published in the journal Monthly Notices of the Royal Astronomical Society Letters.

“The area of ​​stellar spots does not seem to be the main responsible for triggering the superexplosions. Perhaps the explanation should be sought in the magnetic complexity of the active regions”, says Alexandre Araújo, a professor at the Integrated Center for Young People and Adults (Cieja – Campo Limpo) at the City Hall of São Paulo, postdoctoral fellow at the Mackenzie Engineering School and first author of the article.

With support from Fapesp, the study was conducted by him and his former doctoral advisor, current postdoctoral supervisor, Adriana Valio, a researcher at the Mackenzie Center for Radio Astronomy and Astrophysics (CRAAM), at Universidade Presbiteriana Mackenzie.

The sunspots of both stars were characterized with the planetary transit mapping technique, which provides intensity, temperature, position (latitude and longitude) and radius.

“From the knowledge that was available in the literature, stars with larger spots would be more likely to produce superflares, but that is not what we observed. Kepler-411’s star spots are much smaller than those of Kepler-210. In theory, this would be that there should be superexplosions, but that doesn’t happen. Our explanation for the non-existence of superflares on Kepler-210, even with large spots on its surface, lies in the magnetic complexity, evolution and lifetime of the spots”, says Araújo.

In addition to seeking a breakthrough in the knowledge of stellar activities, the present study had an additional motivation. After the discovery of the first superexplosions in solar-type stars, the scientific community began to look closely at such phenomena, mainly to investigate what would be the possibilities for the Sun to present an explosion of this proportion. If eruptions of much lower intensity already have such a strong impact on our technological society, what can we expect from energetic phenomena of such magnitude?

“Certainly the planets that orbit stars with a frequency of superflares can end up losing their atmosphere and, for this reason, not developing life — at least life as we know it”, answers Araújo.

The structure of solar-type stars

To understand all this, it is necessary to open a wide parenthesis and recapitulate some basic knowledge about the structure of stars, obtained mainly from studies on the Sun. For didactic purposes, this structure is divided into layers.

“The core is the star’s main source of energy. In the Sun, this region is a sphere whose radius corresponds to the fifth part of the solar radius, but with extremely high density. In it, the conversion of hydrogen into helium, through thermonuclear reactions , produces a temperature of around 13.6 million kelvin (K)”, informs Valio.

Surrounding the nucleus is the radiative zone, where energy is transported by photons in all directions. Photons, as is known, are the particles associated with electromagnetic radiation. And its speed of propagation in vacuum is the greatest in the material universe. However, as the radiative zone is composed of particles (protons, electrons, etc.), absorption and subsequent emission by these components greatly impede the transit of photons. So it takes them about 1 million years to get through this layer and get to the next one, the convective zone.

“In the convective zone, energy is transported via convective currents. The hotter material rises to the surface of the star, while the colder, denser material sinks back into the convective layer. This movement creates giant cells, which transport energy and material through the star. On the surface of the Sun, they are known as the solar granules”, explains Valio.

The surface of the Sun is called the photosphere. It is there that sunspots, granules and eruptions appear, which extend throughout the solar atmosphere, composed of the chromosphere and corona. The average temperature of the photosphere is just over 5,700 K, which makes it relatively cool compared to the inner layers of the Sun or the upper layers of the solar atmosphere. It is from the photosphere that most of the light and heat emitted by this star come out.

“The spots that appear on the photosphere are caused by intense magnetic fields and can last from a few days to several weeks before disappearing. Their formation begins with a magnetic field generated by the movement of electrically charged particles in the tachocline, a thin layer between the radiative regions and convective from the solar interior. When emerging on the surface of the Sun, the magnetic flux tubes create regions of intense field, which block the transfer of heat from the interior to the surface. The spots are dark because their temperature is 1,000 to 1,500 degrees lower than than the temperature of the rest of the surface”, describes Valio.

And he adds that the spots generally have different shapes and sizes, with their magnetic complexity being a crucial factor in producing the largest solar flares. These are observed throughout the electromagnetic spectrum: radio, infrared, visible light, ultraviolet, X-rays and gamma rays. Such transient phenomena take place in the solar atmosphere, in regions of high magnetic field concentrations, where large amounts of energy are released by magnetic reconnection. The power generated in the largest solar flares is approximately 1017 to 1022 kilowatts.

The method of planetary transits

The great challenge for superflare researchers is to unravel the mechanisms that originate such phenomena. It is consensual that these large explosions are related to star spots. But in what way? “The planetary transit method is excellent for investigating spots on the surface of solar-type stars. This method is currently the most robust for this type of investigation. But its application is quite complicated, mainly due to the difficulty of obtaining stars that fit the criteria. investigation criteria”, comments Araújo.

He and Valio worked with data from the Kepler telescope, looking for stars that fit the study’s profile. The Kepler space telescope was designed by NASA, the US space agency, with the aim of discovering terrestrial planets outside the Solar System. In the four years of its first phase of operation, which lasted from 2009 to 2013, it observed more than 150,000 stars. And, to extract information about these objects, the method of planetary transits was used, which is based on the tiny change produced in the brightness of the star when a planet passes in front of it.

But finding, in that gigantic database, the objects that suited his purposes was, as Araújo said, like looking for a needle in a haystack. He details: “First of all, the star had to have one or more planets. In order for these exoplanets to be detected, their angle of inclination in relation to the star had to be in the telescope’s viewing angle. In addition, the star had to present spots on its surface. And the exoplanet should transit in the regions of spots. The exoplanet’s orbital period had to be a few days. And its radius should be much larger than Earth’s, so that the drop in brightness caused in the curves of light from the star was quite significant. Finally, the star needed to exhibit superflares”.

The researcher states that, fortunately, it was possible to identify a star, Kepler-411, with excellent observation quality. And the best: it had a planetary system with four exoplanets. But to understand the role of stellar spots, it was necessary to find a second star that was similar in everything, except for one aspect: it could not have superflares. “It was, in a way, daring of us to believe that this second star existed. And we felt rewarded when we found Kepler-210, with stellar parameters very close to Kepler-411”, he says.

It is believed that the detection of superflares is directly linked to the temporal coverage of spots on the surface of stars. And that, the greater the area of ​​the stellar spots, the greater the storage of magnetic energy to produce the explosion.

“Our results brought a slightly different perspective. As already mentioned, on Kepler-411, we detected 65 superflares, with energies of up to 1,035 ergs [1.035 ×107 quilojoule]. While Kepler-210 did not show any superflares, even with twice the temporal coverage, which gave us a greater probability of observation. And what surprised us the most was the fact that the radii of Kepler-411’s star spots are much smaller than those of Kepler-210”, emphasizes Araújo.

The explanation may lie in the fact that, despite being larger in area, the spots on Kepler-210 have a simpler magnetic configuration.

“On the Sun, spots are classified according to the behavior of the magnetic field in the area. They are classified as alpha (α), beta (β), gamma (γ) and delta (δ), or through a combination of these configurations . The delta spots are the ones that show intense solar flare activity. We believe that the Kepler-210 spots present a simpler magnetic configuration, of the alpha or beta type. Unfortunately, the exact confirmation of this hypothesis would only be possible through magnetograms, which are images capable of detecting the location and intensity of magnetic fields. Currently, we can only observe this in the Sun. We still don’t have the technology to obtain magnetograms of distant stars. Anyway, our study already allows us to say that, instead of close the focus on the area of ​​stellar spots, perhaps it is more productive to consider the magnetic complexity of the active regions”, concludes Valio.

The article The connection between starspots and superflares: a case study of two stars can be accessed here.