Hydrogen Walls: Mass Loss of Dwarf Stars and the Young Sun

Abstract

Charge-exchange reactions between a fully ionized stellar wind and the partially ionized warm gas in the interstellar medium create a compressed region of hot neutral hydrogen atoms that are decelerated relative to the inflowing interstellar gas. This “hydrogen wall” produces a blue-shifted absorption component in the stellar Lyman-a emission line at many viewing angles that has now been detected in Hubble Space Telescope spectra of eight dwarf stars. Comparisons of the observed Lyman-α line profiles with theoretical models led to the first sensitive measurements of mass-loss rates as small as 4 × 10^-15 M_Θ yr^-1 for solarlike dwarfs. Our study of astrospheres provides the first observational data (other than for the Sun) with which to test theories for dwarf star winds. We find an empirical correlation of stellar mass-loss rate with X-ray surface flux that allows us to predict the mass-loss rates of other stars and to infer the solar wind mass flux at earlier times, when the solar wind may have been as much as 1000 times stronger than at present. We mention some important ramifications for the history of planetary atmospheres in our solar system, that of Mars in particular, and for exoplanets.