Carbon and nitrogen isotope ratios in carbon stars: a new approach.

A method of comparing an observed spectrum with synthetic spectra has been applied to the deduction of isotope ratios in carbon stars. The analysis technique is based on the mutual coherence of the two spectra being compared and is an objective method of evaluating their 'goodness of fit'. This technique is particularly well suited to the analysis of carbon-star spectra since it is not affected by errors in the level of the drawn continuum and does not require that particular lines be selected for analysis; instead, it uses the entire spectrum. By varying the parameters describing the synthetic spectrum, the coherence can be maximized and the isotopic ratio in the stellar spectrum deduced. Results for five carbon stars yield C-12/C-13 ratios in the range 2.5-30. A search was also made for the isotopes C-14 and N-15; C-14 was not found, while a tentatively positive result is reported for N-15.