Observational Estimates for the Mass-Loss Rates of α Centauri and Proxima Centauri Using [ITAL]Hubble Space Telescope[/ITAL] L[CLC]y[/CLC]α Spectra

We study H I Lyman-alpha absorption observed by the Hubble Space Telescopetoward the nearby binary system Alpha Cen (G2 V+K0 V) and its distant companionstar Proxima Cen (M5.5 Ve). Absorption from heliospheric H I heated by thesolar wind/ISM interaction is observed toward both Alpha Cen and Proxima Cen.Absorption from analogous "astrospheric" material surrounding the stars isdetected toward Alpha Cen, but not Proxima Cen. The nondetection ofastrospheric absorption toward Proxima Cen suggests that the stellar wind ofProxima Cen must be significantly weaker than that of the Alpha Cen system. Weuse hydrodynamic models of the astrospheres computed assuming differentmass-loss rates to predict astrospheric Lyman-alpha absorption for comparisonwith the observations. The model that best matches the Alpha Cen data has amass-loss rate of twice the solar rate, and the models suggest an upper limitof 0.2 solar for Proxima Cen. Finally, we note that the heliospheric absorptionobserved toward Proxima Cen in 2000 May is identical to the heliosphericabsorption observed toward Alpha Cen in 1995 May, implying that the structureof the outer heliosphere does not change significantly during the solaractivity cycle.Comment: 11 pages, 3 figures; AASTEX v5.0 plus EPSF extensions in mkfig.sty; accepted by ApJ Letter