  High resolution UV spectra of stellar H I Lyman-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 or comparable in strength to that of the Sun, i.e.
Mdot<=1 Mdot_sun. However, two of the M dwarfs have much stronger winds: YZ CMi
(M4 Ve; Mdot=30 Mdot_sun) and GJ 15AB (M2 V+M3.5 V; Mdot=10 Mdot_sun). 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 Mdot 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.
