The structure and spectra of irradiated secondaries in close binaries (pre-CVs)


Many stars evolve not alone but along one or more companions. While the evolution of single isolated stars is well predicted by theoretical calculations, close binary systems with their interactions are on the one hand far more difficult to handle, but on the other hand well constrained in their masses and sizes. One mechanism thought to produce such close binaries is the Common Envelope Evolution (CEE). Especially young post CEE binaries show great temperature differences between both components of about a factor 10 at a separation of just a few solar radii. This work focuses on close binary systems with a hot primary component (e.g. a subdwarf O-type star (sdO)) and a much cooler secondary component (e.g. a main-sequence star), which is heavily influenced by external radiation originating from the first. The technique to include external radiation into the stellar atmosphere code PHOENIX, i.e., the deviation of the outer boundary condition and the change in the temperature correction method, is described in detail. To concentrate on the irradiation effect, other difficulties such as asymmetric effects due to tear-drop shaped stars, ongoing mass accretion or shadows of a circumbinary disc, are avoided by selecting pre-cataclysmic variables (pre CVs) out of the class of close binaries as laboratory. The total eclipsing binary system UU~Sagittae is used as an example for massive irradiation and observed spectral features in the optical are fitted ...
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