R Arae: an interacting, eclipsing binary star system

Abstract

The purpose of this thesis is to study R Arae (HD 149730A), a bright (~6.5 magnitude), medium period (~4.425 days) southern eclipsing binary star system, to see if it is semi-detached and to investigate the out of eclipse level fluctuations seen in previous studies. Here R Arae was observed using broad-band U, B, V differential photometry and Hβ absolute photometry. The U, B, V light-curves were fitted using Budding's binary star modeller, allowing for the light of the visual companion star (HD 149730B) which had to be included in all observations due to it's small angular separation from R Arae (~3.6 arcseconds). From the results it is concluded that R Arae is a semi-detached Algol system with a main sequence B9 primary and a sub-giant F secondary star, which is undergoing mass transfer from the secondary on to an accretion disk around the primary star. A systematic deviation from the model light-curve around phase 0.8 was found, which is thought to be due to the an impact region of the Roche Lobe overflow stream on to the accretion disk, with the out of eclipse fluctuations being irregularities in the mass transfer rate and/or in-stabilities in the accretion disk. No binary light-curve structure was observed in the Hβ index measurements, with a large scatter being found instead. It is concluded that this scatter is due to a thin Hydrogen cloud surrounding R Arae resulting from the Roche Lobe overflow. Overall, the results and model proposed in this thesis show good agreement with observations made in other investigations over diverse wavelength (visual, ultra-violet, X-ray and spectroscopic) ranges and resolutions. However further observation over a long time base (tens of years) is needed to confirm the above conclusions and to follow the evolution of the accretion disk in this particularly interesting binary system.