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Modeling the Interstellar Medium-Stellar Wind Interactions of λ Andromedae and ϵ Indi

A numerical study is presented of the interaction between the partially ionized interstellar medium (ISM) and the stellar wind of the very active RS CVn-type binary star λ And (G8 IV-III+?). Similar to results found for the heliosphere-ISM interaction, a termination shock, a tangential discontinuity (the "astropause"), and a bow shock are found in the model. More importantly, a hydrogen wall forms in the model, and its neutral density, temperature, and velocity are investigated. As an application, the additional absorption of the Lyα line of λ And due to the hydrogen wall is calculated and compared with high-resolution UV spectra obtained by the Hubble Space Telescope (HST). Variation of stellar wind parameters results in different astrospheres and hydrogen walls, with distinct Lyα absorption. Although not studied here, variation of interstellar parameters will alter the astrospheres as well, just as in the heliospheric case. Thus, establishing a good model fit to the HST data is a tool to constrain the combination of parameters of stellar wind and ambient interstellar medium. A second numerical study is undertaken for the nearby star ϵ Ind (K5 V). Again, a hydrogen wall is encountered, and the correction to interstellar Lyα absorption demonstrated. While ϵ Ind is interesting in that it has a high relative velocity with respect to the ISM, λ And is interesting in that it is a large, extremely active star that we therefore model with a higher mass-loss rate than the Sun. The numerous assumptions that are made in the models preclude a definitive measurement of mass-loss rates for these stars at this point, but our models suggest that for ϵ Ind the observed Lyα absorption can be reproduced adequately by a wind with roughly the solar mass-loss rate. For λ And, a mass-loss rate of roughly 10 times solar is suggested, which is actually lower than one might have expected since the star has about 55 times the surface area of the Sun.