Short-term Hα Variability in M Dwarfs

We study the variability of Hα emission in midto late-M dwarfs on timescales of ∼ 0.1 − 1 hr as a proxy for magnetic activity. We spectroscopically observed 43 active dwarfs with spectral types M3.5–M8.5 for ∼ 1 hr each, and with an exposure cadence of 5–10 minutes. About 80% of the objects exhibit statistically significant variability on the full range of timescales probed by the observations, and with amplitude ratios in the range of∼ 0.1−4. No events with an order of magnitude increase in Hα luminosity were detected, indicating that their rate is . 0.05 hr (95% confidence level). We find a clear increase in variability with later spectral type, despite an overall decrease in Hα “activity” (i.e., LHα/Lbol), as well as a correlation between variability and mean equivalent width. For the ensemble of Hα events, we find a nearly order of magnitude increase in the number of events from timescales of about 10 to 30 min, followed by a roughly uniform distribution at longer durations. The event amplitudes follow an exponential distribution with a characteristic scale of Max(EW)/Min(EW)− 1 ≈ 0.7. This distribution predicts an extremely low rate of ∼ 10 hr for events with Max(EW)/Min(EW) & 10. However, the serendipitous detections of such events in past M dwarf observations suggests that large flares represent a different distribution of events. Finally, we find a possible decline in amplitude for events with durations of & 0.5 hr, which may point to a typical energy release in Hα events (EHα ∼ LHα× t ∼ const). Longer observations of individual active objects are required to further investigate this possibility. Similarly, a larger sample may shed light on whether Hα variability correlates with properties such as rotation velocity. Subject headings: stars: magnetic fields — stars: flare — stars: late-type — stars: activity