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Paper: Semi-empiric Radiative Transfer Modeling of FUSE Stellar Spectra
Volume: 348, Astrophysics in the Far Ultraviolet: Five Years of Discovery with FUSE
Page: 171
Authors: Lobel, A.; Avrett, E.H.; Aufdenberg, J.P.
Abstract: 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 CIII and OVI 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 CIII λ977 (and MgII h & k) observed in spectra of luminous cool stars such as α 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 Ovi resonance emission lines in the supergiant α Aqr, the classical Cepheid variable β Dor, and to self-absorbed OVI emission lines in the cataclysmic variable SW UMa.
We observe that the CIII resonance line profile of α 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 kms−1 at kinetic temperatures of 65 kK and higher. Based on detailed model fits to the narrow red-shifted and self-absorbed OVI 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 propose that the large gas density of ρ≈1.4 10−11 g cm−3 favors a region of warm dense plasma of 100 kK ≤Tgas≤300 kK that collapses onto the white dwarf with a mass accretion rate of 1−2 1015 g s−1 above or between the accretion disk. 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.
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