Short-term Hα Variability in M Dwarfs
We spectroscopically study the variability of Hα emission in mid- to late-M dwarfs on timescales of ~0.1-1 hr as a proxy for magnetic variability. About 80% of our sample exhibits statistically significant variability on the full range of timescales probed by the observations, and with amplitude ratios in the range of ~1.2-4. No events with an order of magnitude increase in Hα luminosity were detected, indicating that their rate is 0.05 hr –1 (95% confidence level). We find a clear increase in variability with later spectral type, despite an overall decrease in Hα "activity" (i.e., L Hα / L bol ). For the ensemble of Hα variability events, we find a nearly order of magnitude increase in the number of events from timescales of about 10-30 minutes, 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 a low rate of ~10 –6 hr –1 for events with Max(EW)/Min(EW) 10, but serendipitous detections of such events in the past suggest that they represent a different distribution. Finally, we find a possible decline in the amplitude of events with durations of 0.5 hr, which may point to a typical energy release in Hα events for each spectral type ( E Hα ~ L Hα × 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 age or rotation velocity.