Further Evidence for Carbon Isotopic Structure in the λλ5797 and 6614 Diffuse Interstellar Bands

Profiles of the λλ5797 and 6614 diffuse interstellar bands (DIBs) in our high-resolution spectra ( R = 120,000-60,000) of five stars provide strong support for the model proposed by Webster, in which 13 C atoms are randomly substituted for 12 C in highly symmetric molecules. These stars have narrow interstellar Na D lines, and the DIB profiles can be reproduced, as predicted, by a band of equally spaced, equally wide Gaussians with a Poisson intensity distribution. Other good fits are possible with components of unequal FWHM, but the widths, positions, and relative amplitudes of the components then differ from star to star, which is less credible for a common absorber. The average numbers of 13 C atoms per molecule derived from the model are about 0.95 and 1.41 for the λλ5797 and 6614 DIBs, respectively, which correspond to between 40 and 100 carbon atoms per molecule. Sharp features in the cores of the λ5797 DIB are not exactly matched by the Gaussians, which implies a more complex intrinsic structure. For HD 37061 in Orion, both DIBs are weak and have a component on the short-wavelength side of the normal profile, possibly indicating higher rotational temperatures.