Short-Term Hα Variability in M Dwarfs

Abstract

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., LHα/Lbol). 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 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 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 suggests 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 (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 age or rotation velocity.