Exploring stellar activity in a sample of active M dwarfs

TL;DR

This work analyzes magnetic activity in active M dwarfs by combining high-cadence TESS photometry with optical spectroscopy to connect flare occurrence, energies, rotation, star-spot filling, and chromospheric indicators. By applying automated flare detection (Altaipony) and energy/FFD analyses, it reveals a near-flat FOR for M0–M4 that declines for cooler types, and shows that rapid rotators ($P_{\rm rot}<1$ d) have significantly enhanced flaring. The flare frequency distributions follow power laws with slopes $\alpha$ increasing from $\sim1.68$ to $\sim1.95$ from M0 to M5, indicating relatively rarer high-energy events in later types; higher FOR correlates with lower flare amplitudes, suggesting energy release in many small flares in more active stars. The study also finds a mass- and rotation-linked rise in chromospheric activity (e.g., H$\alpha$) that coincides with stronger FOR, supporting a cohesive, dynamo-driven framework for activity in fully convective M dwarfs and highlighting implications for exoplanet habitability around these stars.

Abstract

Most M dwarfs show higher chromospheric activity, often exceeding solar levels. Characterizing stellar activity is essential, particularly since these stars are prime targets in the search for habitable exoplanets. We investigate the stellar activity of active M dwarfs using TESS photometry combined with spectroscopic observations. We explore relations between flare occurrence rate (FOR), flare energies, rotation period, starspot filling factor, and chromospheric indicators. We also examine correlations between flare amplitude, duration, and cumulative flare frequency distributions to probe the mechanisms behind magnetic activity. We find that FOR is flat across spectral types M0-M4 but declines for cooler M dwarfs. Rapid rotators ($P_{\rm rot} < 1$ day) display significantly higher FOR and flare activity. M dwarfs with higher FOR tend to have lower flare amplitudes, suggesting that frequent flares are generally less energetic. For stars with 0.15--0.76 $M_\odot$, the median $L_{Hα}/L_{\rm bol}$ varies by a factor of 2.5 across mass bins of 0.1 $M_\odot$, while $Δ$EW decreases by 92\%. The cumulative flare frequency distributions show a decrease in the power-law slope from M0 to M5, with $α$ ranging from 1.68 to 1.95. Our results indicate a transition in stellar activity near M4, where stronger H$α$ emission coincides with higher FOR. We confirm that chromospheric and flare activity follow a power-law relation, highlighting the interplay between magnetic fields and flaring in M dwarfs. We also find that fast rotators sustain frequent flaring through strong dynamos, and that highly active stars dissipate magnetic energy via numerous low-energy flares rather than rare high-energy ones.

Figures (16)

• Figure 1: Distance distribution (top panel) of stars in our sample, binned at 5 pc intervals and TESS magnitude distribution (bottom panel) of the same sample.
• Figure 2: Top panel: TESS light curve of PM J16170+5516 (M2.0) from Sector-25, 2020 (SPOC) with $P_{\mathrm{rot}}$ 1.975 days. The black line shows the light curve with a cadence of 2 min, and the red line the Savitzky–Golay filtered and smoothed curve. Bottom panels: zoomed views of detrended flux highlighting multiple flare events of different magnitudes.
• Figure 3: Temporal variation in the H$\alpha$ line profiles for PM J12142+0037 (M4.0) since the beginning of the first exposure. Dashed lines indicate the pseudo-continuum regions. Equivalent widths are marked to the left of each emission line, and elapsed times since the initial exposure are shown on the right.
• Figure 4: Variability and activity indicators as a function of stellar mass. Top panel: distribution of $\Delta \mathrm{EW} = \max(\mathrm{EW}) - \min(\mathrm{EW})$ for H$\alpha$ emission. The black dashed line shows a linear fit, $\Delta \mathrm{EW}_{\mathrm{H}\alpha} = a(M_\ast) + b$, with $a = 7.71 \pm 1.55$, $b = -7.38 \pm 0.64$. Bottom panel: median values of $\log_{10}(L_{\mathrm{H}\alpha}/L_{\mathrm{bol}})$ for H$\alpha$ emission as a function of stellar mass. Black squares mark the medians in seven equally spaced bins, with scatter quantified using the median absolute deviation (MAD) \ref{['Results:variability']}. Blue circles are scaled by $P_{\mathrm{rot}}$, with larger circles corresponding to longer $P_{\mathrm{rot}}$. The red-filled circles indicate stars with no measured rotation period. Vertical gray lines show the bin sizes.
• Figure 5: Distribution of flare durations for M dwarfs (in minutes) shown across spectral subtypes M0–M6.5