Abstract
Abstract. Charge-exchange reactions between outflowing stellar wind protons and interstellar neutral hydrogen atoms entering a stellar astrosphere produce a region of piled-up-decelerated neutral hydrogen called the hydrogen wall. Absorption by this gas, which is observed in stellar Lyman-α emission lines, provides the only viable technique at this time for measuring the mass-loss rates of F–M dwarf stars. We describe this technique, present an alternative way for understanding the relation of mass-loss rate with X-ray emission, and identify several critical issues.
Highlights
Motivation and techniqueThe measurement of mass-loss rates for F–M dwarf stars and the development of theoretical models to predict these rates have challenged observers and theoreticians for many years
Radiative losses in the chromosphere increase even faster than the mass-loss flux, leading first to Msaturation at a high level and to decreasing Mwith increasing FX. This may explain the increase and decrease of Mwith increasing FX, but the peak Mcorresponding to saturation in these calculations is much larger than is observed and the decrease in Mat very large FX levels was not modeled in detail
The study of cool dwarf star astrospheres has made important progress from the early theoretical models of the heliosphere that first showed the presence of a hydrogen wall (Baranov and Malama, 1993), through the development of sophisticated-hydrodynamic models of stellar atmospheres by Müller, Izmodenov, Zank, and others, to the testing and refinement of these models by comparison with stellar Lymanα observations
Summary
The measurement of mass-loss rates for F–M dwarf stars and the development of theoretical models to predict these rates have challenged observers and theoreticians for many years. The greater divergence of field lines from active regions compensates for the larger mass flux in these field lines leading to the same average solar mass-loss rate even when the surface coverage of active regions changes as indicated by a factor of 10 range in FX This explanation for the nearly constant value of Mover at least a factor of 10 range in FX seen in the solar data likely explains the results for the other Group 1 stars. They found that with increasing magnetic field strength and turbulence in the photosphere (the origin of stellar activity), the reflection of upwardly propagating Alfvén waves is less efficient, leading to an increase in the energy available in the corona for mass loss and X-ray emission. This may explain the increase and decrease of Mwith increasing FX, but the peak Mcorresponding to saturation in these calculations is much larger than is observed and the decrease in Mat very large FX levels was not modeled in detail
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