FUMES. III. Ultraviolet and Optical Variability of M-dwarf Chromospheres

Abstract We obtained ultraviolet and optical spectra for nine M dwarfs across a range of rotation periods to determine whether they showed stochastic intrinsic variability distinguishable from flares. The were observed during the Far Ultraviolet M-dwarf Evolution Survey Hubble Space Telescope program using Imaging Spectrograph. observations taken Apache Point Observatory 3.5 m telescope Dual Spectrograph Gemini South Multi-Object used measure multiple chromospheric lines: Balmer series H α H10 Ca ii K lines. find that after excising flares, these lines vary on order 1%–20% at minute-cadence over course an hour. absolute amplitude was greater faster rotating in our sample. Among five stars which we measured weaker lines, note tentative trend fractional increases higher-order integrated flux emission features formed transition region: N v , Si iv, C iv resonance line doublets, He multiplets. signal-to-noise ratio UV data too low us detect nonflare same scale time cadence as optical. consider mechanisms propose both observational theoretical avenues investigation physical causes chromospheres dwarfs.