Cepheid Pulsation--II: Models Fitted to Evolutionary Tracks

The results of a survey of classical Cepheids are presented. The masses and luminosities of the models in the survey were chosen to correspond to where the evolutionary tracks of 5–9 M⊙ stars cross the Cepheid strip. The effect of helium content on the position of the high T e boundary of the instability strip is examined and it is found that for every 15 per cent increase in helium (by mass) the strip shifts by ~ 600 °K to higher T e . Three different modes of pulsation are investigated for instability—fundamental, first and second modes. The existence of second mode instability is confirmed for three stars all of which have a period less than two days. The presence of secondary bumps in the light and velocity curves was a prominent feature of many of the models. However, in no case did the bump occur (for a given period) at the phase expected from a comparison with observation. The only way in which the phase of the bump can be brought into agreement with observation is for the mass, at a given luminosity, to be reduced by a factor ~ 2 compared to the evolutionary calculations.