2007ApJ...667.1150S

We compute the phase lags between the radial velocity curves and the light curves ΔΦ₁=φ^Vr₁-φ^mag₁ for classical Cepheid model sequences both in the linear and the nonlinear regimes. The nonlinear phase lags generally fall below the linear ones except for high-period models, in which they lie above, and of course for low pulsation amplitudes, in which the two merge. The calculated phase lags show good agreement with the available observational data of normal amplitude Galactic Cepheids. The metallicity has but a moderate effect on the phase lag, while the mass-luminosity relation and the parameters of the turbulent convective model (time-dependent mixing length) mainly influence the modal selection and the period, which is then reflected in the period-ΔΦ₁diagram. We discuss the potential application of this observable as a discriminant for pulsation modes and as a test for ultralow-amplitude (ULA) pulsation.