CNO Isotope Abundances in Interstellar Clouds

Abstract

Isotope abundance determinations have been made to date in at least twenty-three dense clouds scattered throughout the galactic disc. Most of these determinations have been made using two or more of the sixteen observed isotopic species of CO, H₂CO, HCN and CS^†. As will be shown, only a few of the molecular experiments can be interpreted independently. The two most notable of these independent experiments are the comparisons of the 6.2 cm absorption lines of H₂CO and H₂ ¹³CO and 2.6 mm emission lines of C¹⁸O and C¹⁷O. Both of these experiments will be discussed at length in subsequent articles in these proceedings. The difficulty of obtaining single isotope abundance ratios is an experimental one. Most terrestial ratios are very large, ranging up to 2675 for the ratio ¹⁶O / ¹⁷O. These large ratios create an experimental bind. Spectral lines of the more abundant isotopic species must be weak enough to ensure that the line suffers no serious optical depth effects. This requirement often makes the line of the less abundant species impossible to detect. Hence many experiments yield double isotope abundance ratios by comparing the line strengths if two molecules having isotope substitutions of two different elements (e.g., |C¹⁸O|/|¹³CO| which yields (¹²C/¹³C)/(¹⁶O/¹⁸O)).