New Observational Constraints on the Winds of M dwarf Stars

High-resolution UV spectra of stellar H I Ly alpha lines from the Hubble Space Telescope (HST) provide observational constraints on the winds of coronal main-sequence stars, thanks to an astrospheric absorption signature created by the interaction between the stellar winds and the interstellar medium. We report the results of a new HST survey of M dwarf stars, yielding six new detections of astrospheric absorption. We estimate mass-loss rates for these detections and upper limits for nondetections. These new constraints allow us to characterize the nature of M dwarf winds and their dependence on coronal activity for the first time. For a clear majority of the M dwarfs, we find winds that are weaker than or comparable in strength to that of the Sun, i.e., (M) over dot <= (M) over dot(circle dot). However, two of the M dwarfs have much stronger winds: YZ CMi (M4 Ve; (M) over dot = 30 (M) over dot(circle dot)) and GJ 15AB (M2 V+M3.5 V; (M) over dot = 10 (M) over dot(circle dot)). Even these winds are much weaker than expectations if the solar relation between flare energy and coronal mass ejection (CME) mass extended to M dwarfs. Thus, the solar flare/CME relation does not appear to apply to M dwarfs, with important ramifications for the habitability of exoplanets around M dwarfs. There is evidence for some increase in. M with coronal activity as quantified by X-ray flux, but with much scatter. One or more other factors must be involved in determining wind strength besides spectral type and coronal activity, with magnetic topology being one clear possibility.

Automated summary

High-resolution UV spectra from the Hubble Space Telescope provide observational constraints on coronal main-sequence stars' winds, yielding new detections of astrospheric absorption in M dwarf stars. The majority of M dwarfs exhibit weaker winds compared to the Sun, with some exceptions, suggesting factors other than spectral type and coronal activity may influence wind strength, such as magnetic topology.