2000MNRAS.311..370C

We have used the Ultra-High-Resolution Facility (UHRF) at the AAT, operating at a resolution of 0.35km.s^–1 (FWHM), to observe Ki and C₂ absorption lines arising in the circumstellar environment of the post-AGB star HD 56126. We find three narrow circumstellar absorption components in Ki, two of which are also present in C₂. We attribute this velocity structure to discrete shells resulting from multiple mass-loss events from the star. The very high spectral resolution has enabled us to resolve the intrinsic linewidths of these narrow lines for the first time, and we obtain velocity dispersions (b-values) of 0.2-0.3km.s^–1 for the Ki components, and 0.54±0.03km.s^–1 for the strongest (and best defined) C₂ component. These correspond to rigorous kinetic temperature upper limits of 211K for Ki and 420K for C₂, although the b-value ratio implies that these two species do not co-exist spatially. The observed degree of rotational excitation of C₂ implies low kinetic temperatures (T_k~10K) and high densities (n~10⁶ to 10⁷cm^–3) within the shell responsible for the main C₂ component. Given this low temperature, the line profiles then imply either mildly supersonic turbulence or an unresolved velocity gradient through the shell.