Observational estimates for the mass-loss rates of α Centauri and Proxima Centauri using Hubble Space Telescope Lyα spectra 1

We study H I Ly alpha absorption observed by the Hubble Space Telescope toward the nearby binary system alpha Centauri (G2 V + K0 V) and its distant companion star Proxima Centauri (M5.5 Ve). Absorption from heliospheric H I heated by the solar wind/interstellar medium interaction is observed toward both alpha Cen and Proxima Cen. Absorption from analogous astrospheric material surrounding the stars is detected toward alpha Cen but not Proxima Cen. The nondetection of astrospheric absorption toward Proxima Cen suggests that the stellar wind of Proxima Cen must be significantly weaker than that of the alpha Cen system. We compute several hydrodynamic models of the astrospheres assuming different mass-loss rates in order to predict astrospheric Ly alpha absorption for comparison with the observations. The model that best matches the alpha Cen data has a mass-loss rate of (M) over dot = 2 (M) over dot., and the models suggest an upper limit of (M) over dot less than or equal to 0.2 (M) over dot. for Proxima Cen. Finally, we note that the heliospheric absorption observed toward Proxima Cen in 2000 May is identical to the heliospheric absorption observed toward alpha Cen in 1995 May, implying that the structure of the outer heliosphere does not change significantly during the solar activity cycle.