On the evolution of the cataclysmic variable stars from the super soft X-ray sources: AE Aquarii a trial case
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Date
2006-11
Authors
Hsien-Chang, Liu
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Publisher
University of the Free State
Abstract
It is shown that the wide binary system and short white dwarf spin period of AE Aquarii (AE Aqr), i.e. Pspin,1~ 33 s , is perfectly reconcilable with a high mass accretion history of the white dwarf. The long orbital period Porb ~ 9 .88 hr of AE Aqr implies that the binary separation is large enough to accommodate a well-developed accretion disc. The rotation period P spin, 1~ 33 s implies that AE Aqr most possibly evolved through a high mass transfer/ accretion phase, where the white dwarf has been spun-up by accretion torques, possibly by an accretion disc. It has also been suggested that in the past AE Aqr could have been a significant X-ray source and possibly even a super soft X-ray source (SSS). This thesis proposes an investigation of the possible connection between high mass transfer and possible SSS properties. Extending the model calculations related to AE Aqr or cataclysmic variables in general show that the evolved (or slightly evolved), secondary stars filling (or nearly filling) their Roche lobe in systems with large orbital period will drive the highest mass transfer rate, implicitly increasing the probability of potential SSS occurrence. In this situation, the most positive aspect is that the SSS phenomena can be satisfied significantly below the Eddington limit for all filling factors and for all orbital period Porb ≥ 9 hrs . Concerning the spin-up history of AE Aqr, the results suggest that the spin-up timescale, rather that the mass transfer time-scale will determine the allowed duration of the run-away mass transfer phase. The model calculations seem also to confirm the magnetic field strength of the white dwarf B1 ≤ few x 10⁶ G , which is believed to be the limiting value of the field strength in AE Aqr. These model calculations readily agree with our conjecture that AE Aqr evolved through a relative brief but violent high mass accretion phase, where the white dwarf has been spun-up to periods P spin,1≤ 33 sec, during which period the accretion onto the compact white dwarf readily could have sustained stable nuclear burning. In this phase, AE Aqr could have been an extremely bright X-ray source, or SSS. This may be a common phase in the evolution of cataclysmic variables in general.
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Cataclysmic variable stars, Double stars -- Evolution, Dissertation (M.Sc. (Physics))--University of the Free State, 2006