GHRS Observations of Cool, Low-Gravity Stars. VI Mass-Loss Rates and Wind Parameters for M Giants

Abstract

The photon-scattering winds of M-giants absorb parts of the chromospheric emission lines and produce self-reversed spectral features in high resolution {\it HST}/GHRS spectra. These spectra provide an opportunity to assess fundamental parameters of the wind, including flow and turbulent velocities, the optical depth of the wind above the region of photon creation, and the star's mass-loss rate. This paper is the last paper in the series `GHRS Observations of Cool, Low-Gravity Stars'; the last several have compared empirical measurements of spectral emission lines with models of the winds and mass-loss of K-giant and supergiants. We have used the Sobolev with Exact Integration (SEI) radiative transfer code, along with simple models of the outer atmosphere and wind, to determine and compare the wind characteristics of the two M-giant stars, $\gamma$~Cru (M3.5III) and $\mu$~Gem (M3IIIab), with previously derived values for low-gravity K-stars. The analysis specifies the wind parameters and calculates line profiles for the \ion{Mg}{2} resonance lines, in addition to a range of unblended \ion{Fe}{2} lines. Our line sample covers a large range of wind opacities and, therefore, probes a range of heights in the atmosphere. Our results show that $\mu$~Gem has a slower and more turbulent wind then $\gamma$~Cru. Also, $\mu$~Gem has weaker chromosphere, in terms of surface flux, with respect to $\gamma$~Cru. This suggests that $\mu$~Gem is more evolved than $\gamma$~Cru. Comparing the two M-giants in this work with previously studied K-giant and supergiant stars ($\alpha$~Tau, $\gamma$~Dra, $\lambda$~Vel) reveals that the M-giants have slower winds than the earlier giants, but exhibit higher mass-loss rates. Our results are interpreted in the context of the winds being driven by Alfv\'{e}n waves.