H\alpha\ Emission Variability in Active M Dwarfs

Abstract

We use ~12,000 spectra of ~3,500 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 multi-object spectrometer, to probe H\alpha\ variability on timescales of minutes to weeks. With multiple observations for every star examined, we are able to characterize fluctuations in H\alpha emission as a function of activity strength and spectral type. Stars with greater magnetic activity (as quantified by L_H\alpha/L_bol) 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 H\alpha\ structure functions to constrain the timescale of variability. The more active objects with lower variability exhibit a characteristic timescale longer than an hour, likely due to larger, longer lasting heating events, while the less active objects with higher variability have a characteristic timescale shorter than 15 minutes.