A measurement of the interstellar [12C/13C] isotopic abundance ratio toward zeta Oph from observations of the lambda 3957 line of CH+

Until recently no visible wavelength line of a secondary isotopic species of any atom or molecule had been detected in the interstellar medium with the sole exception of the lambda 4232 line of 13CH+ which is seen in absorption against zeta Oph. However, there are several problems with the data obtained from this line chiefly to do with the velocity components of the zeta Oph cloud. The authors therefore decided to observe the lambda 3957 line of CH+ toward zeta Oph. (Auth.)