Exploring short-term stellar activity in M dwarfs: A volume-limited perspective

¹ Dipartimento di Fisica e Chimica, Università di Palermo, Via Archirafi 36, 90128, Palermo, Italy
² Blue Skies Space Italia S.R.L. Via Vincenzo Monti 16, 20123, Milano, Italy
³ INAF, Osservatorio Astronomico di Palermo, Palermo, Italy

^⋆ Corresponding author: gabriele.galletta@inaf.it

Received: 25 November 2024
Accepted: 14 April 2025

Abstract

Context. Flares are a form of stellar activity that occur over short timescales but produce highly energetic outbursts. Studying stellar flares is crucial because they can significantly alter the circumstellar environment by producing intense high-energy radiation. Understanding stellar flares is essential for clarifying the environment in which planets evolve, as flares can influence planetary atmospheres by driving photoevaporation and photochemical processes. M dwarfs are of significant interest due to their high flare activity rates and the potential presence of exoplanets within their habitable zones, whose atmospheres may be influenced by flare-emitted radiation.

Aims. We aimed to define the flaring properties of an unbiased sample of M dwarfs with limited volume. Using data from the Transiting Exoplanet Survey Satellite (TESS), we characterized the frequency, energy distribution, and temporal properties of flares in nearby stars.

Methods. We selected a volume-limited sample of M dwarfs within 10 pc from Earth from the Gaia DR3 catalog. We analyzed TESS light curves using an iterative Gaussian process fitting technique to remove long-term stellar activity signals, enabling the identification and characterization of impulsive flare events. For each flare, we derived the amplitudes, timescales, and total energy emitted.

Results. We analyzed 173 stars and detected 17 229 flares, with 0 to 76 flares per TESS sector. We examined the frequency and energy distribution of stellar flares using three representative stars to illustrate the diversity in flare activity. We observed flares with a minimum energy of ∼10²⁹ erg and typical durations ranging from 2 to 8000 seconds. We modeled the cumulative flare energy distribution using one-slope and two-slope fits, yielding average slopes of –0.79 ± 0.64 and –1.23 ± 1.32, respectively. We defined the Flare Energy Index (GF.01) to characterize the flare frequency and revealed two distinct populations. Fainter stars exhibited fewer high-energy flares, whereas brighter stars exhibited more frequent low-energy flares. We analyzed two highly active stars with the largest number of TESS sectors, G 227-22 and G 258-33, were analyzed over a long time baseline to explore their flare properties and energy distributions.