P.D. Hendry (1997), Ph.D. Thesis, University of Toronto

Abstract:

Maximum entropy Doppler images of VW Cephei produced from seven datasets of simultaneous photometric and spectroscopic data are presented. The observations were obtained at the David Dunlap Observatory from March 11, 1991 to May 13, 1993. The photometric observations were made in the Johnson-Morgan, Kron-Cousins BVRI system, and the spectroscopic observations were made at NaD and H-alpha on alternating nights. The data were reduced using algorithms requiring little or no user intervention. Algorithms for automated dispersion fitting, cosmic ray removal, and telluric line removal are presented in detail. An improvement to the Optimal Spectrum Extraction Algorithm of Horne (1986), allowing better extraction under conditions of high sky background, is presented and used.

The Doppler images were produced using maximum entropy regularisation, fitting geometrical and orbital parameters of the contact binary at the same time as spots using both the spectroscopic and photometric data to constrain the models. Spots were modeled as being 3500 K in temperature, fractionally covering each surface element of the system to different amounts. These models fitted the observational data very well.

The Doppler images indicated the presence of large polar spots on both components. The polar spot on the primary was about 50° in diameter, and was slightly off-centre, while that on the secondary was 30° in diameter and somewhat tenuous. A number of lower latitude features were also present. Spots on the primary were found to migrate around in the direction of the orbital motion in the system. Differential rotation of the primary component was observed, with spots at higher latitudes moving at higher angular rates. The spot distribution on the secondary appeared to be quite stable, with spots congregating at active longitudes, and no organised pattern of migration was observed.

The spot coverage on both components was extremely high. Spots covered 66% of the surface area of the primary and 55% of the secondary. The spot model was modeled after the Mullan (1975) starspot model, but was in agreement with the hot secondary model, since the bolometric flux weighted mean surface temperature of the primary was less than that of the secondary, due to the large numbers of spots.

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