Multiepoch Optical Spectroscopy of the Post-AGB Star HD 161796

The evolutionary rate of the pulsating post-asymptotic giant branch (post-AGB) star HD 161796 was suspected to be high. Spectra of HD 161796 acquired during a time span of 18 yr are analyzed with the main goal of determining the evolutionary increase in temperature and comparing it with the latest post-AGB star evolutionary models. Inspection of the spectra reveals splitting and significant temporal variation in strong absorption lines, suggesting the presence of shock waves in the atmosphere of the pulsating star. The H alpha profiles point to variable incipient mass loss. Most medium-strength lines have variable blue wings, while the red wings remain stationary, presumably due to variations in the warm outflow from the stellar surface. The modeling of the spectra suggests the average value for the effective temperature to be 7275 K, and for surface gravity, a value of log g = 0.7. Different iron abundances are found for different spectra, probably due to the inability to model the pulsating photosphere with stationary atmospheric models. On average, we arrive at [Fe/H] = -0.06. The observed underabundance in neutron capture and some other elements is inferred to be a consequence of dust-gas separation. It is confirmed that, during pulsation, the stellar surface is hotter when the star is smaller in size. The spectra show a 420 K range in effective temperature-a smaller variation than can be found from pulsation-related changes in color. No significant rate of evolution is seen, contrary to earlier suggestions. The initial mass of the star is evaluated to be <2 M (circle dot).