H\alpha\ Emission Variability in Active M Dwarfs

TL;DR

This study analyzes H extalpha\ emission variability in active M dwarfs over minutes to weeks using extensive spectral data, revealing that more active stars are less variable and exhibit longer characteristic variability timescales.

Contribution

It provides the first large-scale characterization of H extalpha\ variability in M dwarfs across multiple timescales, linking activity levels to variability patterns.

Findings

More active stars are less variable at all spectral types.

Active stars have longer characteristic variability timescales.

Less active stars show rapid variability with timescales under 15 minutes.

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.