Pulsation in the lower Cepheid strip. I - Linear survey

A detailed linear survey of stellar models intended to represent dwarf Cepheids and Delta Scuti stars has been calculated. Growth rates, periods, and other parameters for the first six radial modes are given for models with X = 0.7, Z = 0.005, and masses ranging from 0.2 to 2.0 solar masses. A complete survey is also presented for the case X = 0.7, Z = 0.02, M = 2.0 solar masses. Although no mass or mode is excluded, observations are consistent with the simple assumption of post-mainsequence Population I evolutionary phase in nearly every case (including period ratios). The fourth and fifth overtones are found to be the most unstable modes in most cases; these modes exhibit both blue and red edges consistent with the observed instability strip. The higher-mass models show a pulsational driving zone at 150,000 K in addition to the usual driving zones at 12,000 K (H) and at 52,000 K (He). This additional driving is generated by an opacity bump due to the second helium ionization edge and is shown to be most effective in models with small central condensation.

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