The 12C/13C-ratio in cool carbon stars

TL;DR

This paper measures the 12C/13C isotope ratios in cool carbon stars using millimetre-wave observations and radiative transfer modeling, highlighting the importance of detailed analysis over simple line ratio methods.

Contribution

It introduces a radiative transfer model to accurately determine stellar 12C/13C ratios from circumstellar 13CO line emissions, improving upon previous methods.

Findings

Derived 12C/13C ratios agree with previous studies.

Optical depth effects complicate simple ratio estimates.

Detailed modeling yields more reliable isotope ratios.

Abstract

We present observations of circumstellar millimetre-wave 13CO line emission towards a sample of 20 cool carbon stars. Using a detailed radiative transfer model we estimate the circumstellar 12CO/13CO-ratios, which we believe accurately measure the important stellar 12C/13C-ratios. For those optically bright carbon stars where it is possible, our derived 12C/13C-ratios are compared with the photospheric results, obtained with different methods. Our estimates agree well with those of Lambert et al. (1986). It is shown that a straightforward determination of the 12CO/13CO-ratio from observed line intensity ratios is often hampered by optical depth effects, and that a detailed radiative transfer analysis is needed in order to determine reliable isotope ratios.