Mullard Space Science Laboratory

M. K. Harrop-Allin
Accretion flows in polars
1999 (Supervisor: M. S. Cropper)

This thesis investigates the accretion flow in polars: close binaries in which a red dwarf transfers matter to a strongly magnetic, synchronously rotating white dwarf.

After an introductory chapter to establish the scientific context, I present a detailed review of our current perceptions of the accretion flow in polars, ending with a list of unanswered questions regarding the nature of the flow.

This is followed by an infrared spectroscopic study of V1309 Orionis. The K-band continuum is dominated by cyclotron radiation from the accretion region on the white dwarf. I use models of the cyclotron continuum to deduce the bolometric luminosity of the system and hence the mass transfer rate through the stream.

I then develop a method to construct images of the accretion stream in eclipsing polars using photometric eclipse profiles. The optimization technique incorporates a genetic algorithm to maximize the chances of finding the global optimum in the multi-dimensional parameter space. The method is tested using synthetic data.

The indirect imaging technique is then applied to high accretion state UBVR data of HU Aquarii. The modelling procedure provides estimates of the radius at which the accretion stream threads onto the magnetic field and hence the mass tranfer rate in the stream. I also investigate the wavelength dependence of emission from different sections of the stream and its implications for the temperature structure along the stream.

To explore changes in the accretion flow as a function of the overall accretion rate, I examine simultaneous intensity and polarization data of HU Aquarii obtained in a low state. The eclipses are modelled using Stokes Imaging, another genetic algorithm-based technique that uses cyclotron models to reconstruct the accretion region on the white dwarf.

The thesis concludes with a discussion of the main results. The results are compared to the questions raised in the review chapter in order to identify the issues that require further investigation.


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