Julien Poyatos | Searcharxiv

arXiv2026

Extending TESS flare frequency distributions with CHEOPS: Power-law versus lognormal

Stellar flares are intense bursts of radiation caused by magnetic reconnection on active stars. They are especially frequent on M dwarfs, where they can significantly influence the habitability of orbiting planets. Flare frequency distributions (FFDs) are typically modelled as power laws. However, recent studies challenge this assumption and propose alternatives such as lognormal laws that imply different flare generation mechanisms and planetary impacts. This study investigates which statistical distribution best describes flare occurrences on M dwarfs, considering both equivalent duration (ED), directly measured from light-curve photometry, and bolometric energy, relevant for physical interpretation and habitability. We analysed 110 M dwarfs observed with TESS and CHEOPS, detecting 5620 flares. We decomposed complex events, corrected for detection biases in recovery rate and energy estimation, and scaled the FFDs to construct a combined distribution spanning six orders of magnitude in bolometric energy. We find that ED-based FFDs follow a power law, reflecting intrinsic photometric flare occurrence. However, bolometric-energy-based FFDs deviate from a pure power law. They are better described by a lognormal distribution, although the best fit is a truncated power law with a break at $1.8 \times 10^{35}$ erg. Using right-tail-stabilised Kolmogorov-Smirnov and exceedance tests, we attribute this deviation to limited sampling of the most energetic events. Our results show that the low-energy flattening, previously interpreted as lognormal behaviour, arises from observational biases and can be corrected through flare injection-recovery and combining observations with different sensitivities. Current instruments cannot reliably sample flares above $10^{35}$ erg, the most relevant for exoplanetary atmospheres. The upcoming PLATO mission will be able to investigate both regimes.

astro-ph.SR↗

arXiv2023

Observing M Dwarfs UV and optical flares from a CubeSat and their implications for exoplanets habitability

M dwarfs show the highest rocky planet occurrence among all spectral types, in some instances within the Habitable Zone. Because some of them are very active stars, they are often subject to frequent and powerful flaring, which can be a double-edged sword in regard of exoplanet habitability. On one hand, the increased flux during flare events can trigger the chemical reactions that are necessary to build the basis of prebiotic chemistry. On the other hand, sufficiently strong flares may erode exoplanets' atmospheres and reduce their UV protection. Recent observations of flares have shown that the flaring flux can be x100 times stronger in UV than in the optical. UV is also preferable to constrain more accurately both the prebiotic abiogenesis and the atmospheric erosion. For these reasons, we are developing a CubeSat payload concept to complement current flare surveys operating in the optical. This CubeSat will observe a high number of flaring M dwarfs, following an all-sky scanning law coverage, both in the UV and the optical to better understand the different effective temperatures as wavelengths and flaring status go. This will complement the bright optical flares data acquired from the current ground-based, high-cadence, wide FoV surveys. Another scientific planned goal is to conduct few-minute after-the-flare follow-up optical ground-based time-resolved spectroscopy, that will be triggered by the detection of UV flares in space on board of the proposed CubeSat. Finally, the study of M dwarfs stellar activity in the UV band will provide useful data for larger forthcoming missions that will survey exoplanets, such as PLATO, ARIEL, HabEx and LUVOIR.

astro-ph.IM↗