Driving Mechanisms for Cataclysmic Variable Evolution.

Barker, John (2003). Driving Mechanisms for Cataclysmic Variable Evolution. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.0000f56c


Cataclysmic variables are interacting binary systems in which the evolution of the system is driven by the loss of orbital angular momentum. In this thesis I investigate possible angular momentum loss mechanisms and try to reconcile the differences between the observed and theoretically predicted minimum period and the orbital period distribution for systems with orbital periods below the period gap. Specifically I use a general consequential angular momentum loss mechanism (CAML) which depends linearly upon the mass transfer rate in the system, and a deformation mechanism which bloats the donor star. Numerical models to include the effects of circumbinary discs and irradiation driven winds from the donor star on the evolution of CVs were developed, the circumbinary disc model is able to raise the minimum period to the observed value. Systems subject to irradiation driven winds and high CAML efficiencies exhibit mass transfer cycles; these could explain the range of mass transfer rates observed in CVs with similar orbital periods. I also consider the possibility that the observed minimum period is purely an age effect.
I model possible parent distributions by using an additional intrinsic angular momentum loss to set the minimum period to the observed value, with different spectra of donor star masses, white dwarf masses, efficiencies for the CAML and bloating mechanisms. A statistical test was developed and used to calculate a probability that the observed distribution is drawn from the modelled parent distribution. None of the calculated distributions gives a better fit than that for a flat distribution. This is suggestive of some additional evolutionary mechanism or selection effect.
I also investigate the apparent difference between the distributions of magnetic and non-magnetic CVs over the period range (1.3 ≤ P/hr ≤ 15), concluding that it is likely that these systems evolve via different mechanisms for orbital periods above the lower edge of the period gap.

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