Carbon stars : absolute magnitudes and carbon and nitrogen isotope ratios

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 ofEarth, Ocean and Atmospheric Sciences, Department ofGraduat