The cool hydrogen-deficient carbon stars and their atmospheres
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Photoelectric photometry of a large sample of R CrB and hydrogen-deficient carbon stars was obtained over a period of five months in order to search for variability and determine the period if variable. All the stars in the sample were found to be variable. Only for the stars S Aps, U Aqr and V CrA were sufficient observations obtained to enable periods to be identified. The determined periods were 39.7, 41.8 and 69.0 days respectively. These periods are in agreement with the theoretical period-temperature relationship. Photoelectric photometry of the hot hydrogen-deficient star DY Cen was obtained over a period of four weeks. DY Cen was confirmed to be variable and the dominate period of 3.8 days determined. This period was consistent with the period-temperature relationship. Model atmospheres were calculated for hydrogen-deficient compositions with temperatures between 5000-8000K and surface gravities between 0.0 and 4.4. The models included the effects of molecular formation, convection and line-blanketing. It was shown that the temperature structure was strongly dependent on the composition, in particular the ratios of C/He and H/He. R CrB was re-analysed using these new models. The derived atmospheric parameters were T[subscript(eff)] = 7400 ± 500K, log g = 0.55 ± 0.25, ξ[subscript(t)] = 8 ± 2kms⁻¹ and C/He=0.005. High resolution spectra were obtained of RY Sgr in order to do a similar analysis. The derived parameters were T[subscript(eff)] = 7000 ± 500K, log g = 0.65 ± 0.25, ξ[subscript(t)] = 10 ± 2kms⁻¹ and C/He=0.005. Both stars were found to have solar metallicities with no over-abundances of s-process elements. The abundances of C, N and O were all enhanced relative to the solar values. Medium resolution spectra were obtained at the Isaac Newton telescope of suspected R CrB stars in order to correctly classify them. The stars were classified on the basis of the strength of the hydrogen lines and the G band. BG Cep, LO Cep, CC Cep, DZ And, RZ Vul, VZ Vul, V638 Her and V1405 Cyg were all classified as not being R CrB stars. UV Cas, SU Tau and SV Sge were classified as R CrB stars.
Thesis, PhD Doctor of Philosophy
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