Semi-empiric Radiative Transfer Modeling of FUSE Stellar Spectra

We present an overview of radiative transfer modeling efforts to interpret spectra of a variety of stellar objects observed with FUSE. Detailed radiative transfer modeling of high ion emission line profiles of C III and O VI observed in the far-UV spectrum provides a powerful means to probe the thermal and dynamic properties of high-temperature plasmas in the atmospheres of stars. We model asymmetric emission lines of C III lam977 observed in spectra of luminous cool stars such as Alpha Aqr, to infer the wind- and microturbulence velocity structures of the upper chromosphere. Semi-empiric radiative transfer models that include transition region temperature conditions, are further developed based on detailed fits to O VI resonance emission lines in the supergiant Alpha Aqr, the classical Cepheid variable Beta Dor, and to self-absorbed O VI emission lines in the cataclysmic variable SW UMa. We observe that the C III resonance line profile of Alpha Aqr assumes a remarkable asymmetric shape, reminiscent of P Cygni type profiles observed in hot luminous supergiants. The model calculations indicate outflow velocities above ~140 km/s at kinetic temperatures of 65 kK and higher. Based on detailed model fits to the narrow red-shifted and self-absorbed O VI emission lines of SW UMa we compute that the gas- and electron-density exceed the density conditions of the upper solar transition region by about three orders of magnitude. We discuss how detailed semi-empiric fits to emission lines observed with the high spectral resolution of FUSE can provide reliable constraints on the mass-loss or mass-accretion rates in these objects.