Ultra-high-resolution measurements of the intrinsic line profiles of interstellar C-2 towards zeta Ophiuchi and HD 169454

We have used the Ultra-High-Resolution Facility at the Anglo-Australian Telescope (AAT), operating at a resolution of 0.35 km s(-1) (FWHM), to measure the intrinsic profiles of the Q(2) and Q(4) lines of the (2-0) Phillips band of interstellar C-2 towards zeta Oph and HD 169454. The C-2 lines were found to be very narrow, with intrinsic velocity dispersions (b-values) in the range 0.25-0.71 km s(-1) In the case of zeta Oph, two velocity components (separation 1.10 +/- 0.17 km s(-1)) have been resolved, and found to have rather different linewidths and rotational excitation temperatures. In the case of HD 169454, the data are consistent with a single velocity component, but the well-defined b-value (0.59 +/- 0.04 km s(-1)) and low kinetic temperature would then imply supersonic turbulent velocities. One way to avoid this conclusion is to postulate the existence of unresolved velocity structure (or a velocity gradient) within the cloud.