Statistical Signatures of Nanoflare Activity. III. Evidence of Enhanced Nanoflaring Rates in Fully Convective stars as Observed by the NGTS

TL;DR

This study investigates nanoflare activity in M-dwarf stars, revealing that fully convective stars exhibit significantly higher nanoflare rates and distinct decay timescales compared to partially convective stars, with implications for stellar magnetic reconnection.

Contribution

It provides new statistical and Fourier analysis evidence showing enhanced nanoflare activity specifically in fully convective M-dwarf stars, linked to their magnetic reconnection processes.

Findings

Fully convective M-dwarfs have higher nanoflare rates.

Decay timescales differ between stellar sub-types.

Enhanced small-scale magnetic reconnection in fully convective stars.

Abstract

Previous examinations of fully-convective M-dwarf stars have highlighted enhanced rates of nanoflare activity on these distant stellar sources. However, the specific role the convective boundary, which is believed to be present for spectral types earlier than M2.5V, plays on the observed nanoflare rates is not yet known. Here, we utilize a combination of statistical and Fourier techniques to examine M-dwarf stellar lightcurves that lie on either side of the convective boundary. We find that fully convective M2.5V (and later sub-types) stars have greatly enhanced nanoflare rates compared with their pre-dynamo mode transition counterparts. Specifically, we derive a flaring power-law index in the region of $3.00 \pm 0.20$, alongside a decay timescale of $200 \pm 100$~s for M2.5V and M3V stars, matching those seen in prior observations of similar stellar sub-types. Interestingly, M4V stars exhibit longer decay timescales of $450 \pm 50$~s, along with an increased power-law index of $3.10 \pm 0.18$, suggesting an interplay between the rate of nanoflare occurrence and the intrinsic plasma parameters, for example, the underlying Lundquist number. In contrast, partially convective (i.e., earlier sub-types from M0V to M2V) M-dwarf stars exhibit very weak nanoflare activity, which is not easily identifiable using statistical or Fourier techniques. This suggests that fully convective stellar atmospheres favor small-scale magnetic reconnection, leading to implications for the flare-energy budgets of these stars. Understanding why small-scale reconnection is enhanced in fully convective atmospheres may help solve questions relating to the dynamo behavior of these stellar sources.