Carbon stars are relatively uncommon, luminous, cool stars whose spectra exhibit exceptionally strong bands of carbon-containing molecules. This is direct evidence of extensive nucleosynthesis, as will occur in the late stages of stellar evolution. The two aspects investigated here are their luminosities and atmospheric carbon and nitrogen isotope ratios. The luminosities are derived through the study of those carbon stars which are members of double star systems. Since the companion star is apparently normal and thus of known luminosity, the carbon star luminosity is directly attainable. Photometry and spectroscopy of suspected binaries yield absolute visual magnitudes for a dozen stars as bright as -4.7, and bolo-metric magnitudes primarily in the range -4 to -8. This means they are slightly mere luminous than normal giants. The isotopic ratios have been deduced by a comparison of synthetic spectra with the observed near infrared stellar spectra. The synthetic spectra were calculated by direct integration of the flux emerging from an appropriate model atmosphere, and contain lines primarily of the Bed band system of the CN molecule. A new analysis technique, used in time series analysis, which is based on the mutual coherence of the spectra being compared and makes full use of the entire spectrum, has been employed. By varying the parameters describing the synthetic spectrum, including the isotopic ratio, the coherence can be maximized and the isotopic ratio of the stellar spectrum deduced. Results for five carbon stars yield ¹²C/¹³C ratios in the range 2.5 to 30. A search was also made for the isotopes ¹⁴C and ¹³N; ¹⁴C was not found, while a tentatively positive result is reported for ¹⁵N. / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Graduate

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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 (Ph. D.)--University of Florida, 1982. / Description based on print version record. Typescript. Vita. Includes bibliographical references (leaves 95-97).

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