The characteristics of cool hydrogen deficient carbon stars
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Observations of many R Coronae Borealis (RCB) stars and Hydrogen deficient Carbon (HdC) stars have been obtained to more fully understand the characteristics of these types of objects. A long term photometric programme has been undertaken to observe most of the known RCB and HdC stars, including those in the Large Magellanic Cloud. Photometry has been obtained over durations of 700-1100 d for many of these stars, permitting a more accurate analysis of the light and colour curves of these objects than has been possible with previously published data. Properties of the low amplitude variations, observed in all these stars, are described and analysed using Fourier techniques. By combining the observed periods and estimated temperatures, the latter obtained from spectra, a period temperature relationship is produced for these stars. Most stars have a period that can be identified with a radial pulsation mode from models of hydrogen deficient carbon stars. Theoretical period decrease rates for these types of stars indicate that cooler (T eff ≈ 5000 K) stars should dominate the temperature distribution. This is not observed. The reasons for this apparent lack of cooler stars are discussed. If the evolutionary models are correct, there should be many more cool hydrogen deficient carbon stars still to be discovered. Observations obtained during declines of several RCB stars are discussed. The observations of the 1988 decline of R CrB represent the first occasion where both photometric and spectroscopic observations were obtained during the onset of the decline. For those declines for which extensive photometry is available, a major division in photometric properties is observed during the early stages of the decline. This division, where the colours of the star become either bluer, or redder, may be due to a range of dust formation angles, which alters the relative flux contributions of the photosphere and chromosphere of the star. Additional aspects of two RCB stars, NSV 6708 and RY Sgr, are described in greater detail. For NSV 6708, the low radial velocity amplitude of NSV 6708 (< 4 kms-¹) causes some difficulty for the standard decline model for these stars, which assumes that the dust formation is linked to radial pulsations. For RY Sgr, observed changes in the pulsation period in the star are interpreted with an O-C analysis to the times of maxima. Only a series of linear solutions, which have lifetimes of ~ 10 yrs, satisfy the O-C residuals to within the observational uncertainty. In addition, spectra have been obtained during the shock wave event of RY Sgr, which is seen as a splitting of the absorption lines near Vmax. Low excitation lines have a greater shock amplitude, which is consistent with their formation in the upper regions of the photosphere of the highly extended atmospheres of these stars. Finally, areas where additional progress could be made on the study of these stars are discussed.