Other
Scientific paper
May 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006phdt.........4c&link_type=abstract
PhD Thesis, Trinity College Dublin, 2006
Other
6
Symbiotic Stars, Ultraviolet, Mass Loss, Individual (Eg Andromedae), Chromospheres, Line Identification
Scientific paper
The mass lost from red giant stars, in the form of a relatively dense and slow-moving wind, enriches the interstellar medium with much of the material that has been processed inside the star. However, despite the importance and ubiquity of these winds much remains unknown about the outflow conditions and characteristics. For isolated giant stars, the diagnostics that can be obtained from observations are limited, with only disk-averaged information being directly observable. Spatially resolved observational constraints within the wind acceleration zone are required. This thesis presents a series of 20 FUSE and HST/STIS observations of the bright eclipsing symbiotic binary system EG Andromedae. The system consists of a low-luminosity white dwarf and a mass-losing, non-dusty M2.4 red giant star. The ultraviolet observations follow the white dwarf continuum through periodic eclipses by the wind and chromosphere of the giant, providing a unique, spatially resolved diagnosis of the circumstellar gas in absorption against the attenuated dwarf continuum. Analysis of high and low-resolution optical spectra shows that the atmospheric structure of the giant star is not severely perturbed, either radiatively or tidally, by the presence of the binary companion.
The ultraviolet spectra are dominated by effects associated with the eclipse of the dwarf by the giant atmosphere and wind. The uneclipsed spectra are dominated by the dwarf continuum with emission lines from an ionised portion of the giant wind superimposed on the continuum. The high ionisation features, such as the O VI resonance doublet (variable on hourly time-scales), diagnose the hot gas close to the dwarf component. During complete eclipse of the hot star, the spectrum is dominated by emission lines originating from both the giant chromosphere and the extended photoionised section of the cool wind. Spectra observed at stages of partial eclipse display a host of low-ionisation, narrow absorption lines, with transitions observed from lower energy-levels up to 5 eV above ground. The terminal wind velocity is found to be 75 km s-1 and the mass-loss rate to be on the order of 10-8 M_sun yr-1. The wind is found to be isothermal throughout the region that is probed, with a derived Fe II excitation temperature of 8,000 K. The ionisation level along each line of sight is observed to be constant and symmetric around eclipse and hydrogen remains predominately neutral. The absorption spectra are modelled successfully assuming collisional excitation and over 4,000 lines are identified and modelled. No molecular features are observed in the wind acceleration region despite the sensitivity of FUSE to molecular hydrogen, precluding molecular opacity being a contributor to the acceleration of the wind. H I column densities are used to derive a wind velocity profile that is incompatible with a low-order beta law and implies a delayed onset of acceleration. This result is confirmed by a photoionisation analysis of each sightline. The small-scale structure of the wind is found to be clumpy, with the flocculi having scale dimensions of a fraction of a solar radius. Further analysis shows that the wind acceleration is unrelated to the presence of dust or molecular/atomic opacity, but is likely to be related to photospheric pulsations.
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