| Short-Term H-alpha Variability in M Dwarfs (2009) | |||||||||
Abstract | |||||||||
| We study the variability of H-alpha emission in mid- to 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 H-alpha 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 H-alpha "activity" (i.e., L_{H\alpha}/L_{bol}), as well as a correlation between variability and mean equivalent width. For the ensemble of H-alpha 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^-6 / 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 H-alpha events (E_{H\alpha} ~ L_{H\alpha} * t ~ const). Longer observations of individual active objects are required to further investigate this possibility. Similarly, a larger sample may shed light on whether H-alpha variability correlates with properties such as rotation velocity.. Comment: Submitted to ApJ; 16 pages, 10 figures, 2 tables | |||||||||
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