Observational Estimates for the Mass-Loss Rates of α Centauri and Proxima Centauri Using Hubble Space Telescope Lyα Spectra

We study H I Lyα absorption observed by the Hubble Space Telescope toward the nearby binary system α 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 α Cen and Proxima Cen. Absorption from analogous "astrospheric" material surrounding the stars is detected toward α 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 α Cen system. We compute several hydrodynamic models of the astrospheres assuming different mass-loss rates in order to predict astrospheric Lyα absorption for comparison with the observations. The model that best matches the α Cen data has a mass-loss rate of = 2 ☉, and the models suggest an upper limit of ≤ 0.2 ☉ 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 α Cen in 1995 May, implying that the structure of the outer heliosphere does not change significantly during the solar activity cycle.