Ultra-high-resolution observations of circumstellar KI and C-2 around the post-AGB star HD 56126

We have used the Ultra-High-Resolution Facility (UHRF) at the AAT, operatin g at a resolution of 0.35 km s(-1) (FWHM), to observe K I and C-2 absorptio n lines arising in the circumstellar environment of the post-AGB star HD 56 126. We find three narrow circumstellar absorption components in K I, two o f which are also present in C-2. We attribute this velocity structure to di screte shells resulting from multiple mass-loss events from the star. The v ery high spectral resolution has enabled us to resolve the intrinsic linewi dths of these narrow lines for the first time, and we obtain velocity dispe rsions (b-values) of 0.2-0.3 km s(-1) for the K I components, and 0.54 +/- 0.03 km s(-1) for the strongest (and best defined) C-2 component. These cor respond to rigorous kinetic temperature upper limits of 211 K for K I and 4 20 K for C-2, although the b-value ratio implies that these two species do not co-exist spatially. The observed degree of rotational excitation of C-2 implies low kinetic temperatures (T-k approximate to 10 K) and high densit ies (n approximate to 10(6) to 10(7) cm(-3)) within the shell responsible f or the main C-2 component. Given this low temperature, the line profiles th en imply either mildly supersonic turbulence or an unresolved velocity grad ient through the shell.