Hα Emission Variability in Active M Dwarfs

We use ~12,000 spectra of ~3500 magnetically active M0-M9 dwarfs from the Sloan Digital Sky Survey taken at 10-15 minute intervals, together with ~300 spectra of ~60 M0-M8 stars obtained hourly with the Hydra multiobject spectrometer, to probe Hα variability on timescales of minutes to weeks. With multiple observations for every star examined, we are able to characterize fluctuations in Hα emission as a function of activity strength and spectral type. Stars with greater magnetic activity (as quantified by LHα/Lbol) are found to be less variable at all spectral types. We attribute this result to the stronger level of persistent emission in the high-activity stars, requiring a larger heating event in order to produce measurable variability. We also construct Hα structure functions to constrain the timescale of variability. The more active objects with lower variability exhibit a characteristic timescale longer than 1 hr, likely due to larger, longer lasting heating events, while the less active objects with higher variability have a characteristic timescale shorter than 15 minutes.