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

We study H I Lyα absorption observed by the Hubble Space Telescope toward the nearby binary system α Cen (G2 V+K0 V) and its distant companion star Proxima Cen (M5.5 Ve). Absorption from heliospheric H I heated by the solar wind/ISM 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 use hydrodynamic models of the astrospheres computed assuming different massloss rates 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 Ṁ = 2 Ṁ⊙, and the models suggest an upper limit of Ṁ ≤ 0.2 Ṁ⊙ 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. Subject headings: hydrodynamics — stars: individual (α Cen, Proxima Cen) — stars: winds, outflows — ultraviolet: ISM — ultraviolet: stars