Session 67 - Stars: Evolution, Atmospheres, Intrinsic.
Display session, Thursday, June 11
Atlas Ballroom,

## [67.07] Wind Acceleration in Evolved Late-Type Stars: The Fe II Wind Problem

G. M. Harper (U. Colorado)

Wind absorption features in UV Fe II emission lines of evolved late-type stars show the dependence of wind velocity on mass-column density. We are constructing detailed semi-empirical chromosphere and wind models for \lambda Vel (K4 Ib) using medium and high resolution HST/GHRS spectra. These models are being used to infer the wind velocity as a function of radius and thereby elucidate the nature of the, as yet unknown, mass-loss mechanisms.

Synthetic line profiles are computed using a model for Fe II with 769 fine-structure levels, approximate net radiative brackets for the rate equations, and with an exact formal solution. Our initial thermodynamic model is scaled from the spectral proxy \zeta Aurigae (K4 Ib) and the wind ionization balance is constrained using new radio observations. Numerical experiments have revealed which Fe II multiplets are the most sensitive diagnostics for constraining the wind acceleration. Spectra with \sim 3\>km\>s^-1 resolution and which cover most of the Fe II emission multiplets are needed to remove potential uncertainties in the interpretation of the Fe II wind features. These requirements are attainable with HST/STIS.