Mass of the stars and intensity of their flares. The study: “Exploring short-term stellar activity in M dwarfs: A volume-limited perspective” of G. Galletta (UNIPA) appeared on A&A

A total of 17229 flares were observed and studied in 173 M-type stars located within 33 light-years from Earth. The study reveals that the most energetic flares occur in the least massive stars, while in the more massive ones, variability is dominated by less intense flares.

Magnetic field generation in stars takes place in their interiors, through complex physical processes primarily dependent on internal structure, convective energy transport, and stellar rotation. These are, of course, regions we cannot observe directly (with the exception of the few stars whose internal structure can be studied via asteroseismology).
So, how can we investigate stellar magnetic fields and relate them to the stellar internal properties?

One solution is to observe transient phenomena occurring in stellar atmospheres, which result from the interaction between local magnetic fields and the plasma in the photosphere, chromosphere, and corona. In particular, stellar flares are rapid events involving the sudden release of energy in the form of radiation, emitted across virtually all wavelengths of the electromagnetic spectrum.
Flares show a wide range in both duration and intensity, in terms of peak energy and total energy emitted over the full event. Studying the statistics of stellar flares and their connection to stellar properties provides valuable insight into the internal structure of stars and their relation with the processes responsible for magnetic field generation.

Moreover, the intense radiation emitted during the most powerful flares can have significant effects on the atmospheres of planets in close orbit around the star.

A team of researchers led by astrophysicist G. Galletta (University of Palermo and Blue Skies Space Italia S.R.L.) analyzed data from NASA’s TESS satellite for 173 M-type stars located within 33 light-years of Earth. TESS is a satellite designed to search for exoplanets and provides light curves (i.e., continuous measurements of stellar brightness over long periods) for stars across almost the entire sky. This makes it a valuable tool for studying the phenomena responsible for stellar variability.

The team focused on M-type stars both because of their intense magnetic activity and because their habitable zone—the region where rocky planets can maintain liquid water on their surface—is located very close to the star, increasing the impact of strong flares on potentially habitable planets.

In total, the team analyzed 17229 flares, discovering that the most energetic ones typically occur in low-mass M stars. By contrast, more massive M stars tend to show short-term variability dominated by less energetic flares.

The study is described in the paper: “Exploring short-term stellar activity in M dwarfs: A volume-limited perspective“, recently published in Astronomy & Astrophysics, in collaboration with astronomers S. Colombo, L. Prisinzano, and G. Micela from our observatory.

The cover figure (click here to view it in full) shows the energy distribution of the flares analyzed in this study, plotted against the bolometric luminosity of the stars, ranging from 0.0001 to 0.1 times the luminosity of the Sun.