The Cepheid Phase Lag Revisited

We compute the phase lags between the radial velocity curves and the lightcurves $\Delta \Phi_1= \phi^{V_r}_1 - \phi^{mag}_1$ for classical Cepheid modelsequences both in the linear and the nonlinear regimes. The nonlinear phaselags generally fall below the linear ones except for high period models wherethey lie above, and of course for low pulsation amplitudes where the two merge.The calculated phase lags show good agreement with the available observationaldata of normal amplitude Galactic Cepheids. The metallicity has but a moderateeffect on the phase lag, while the mass-luminosity relation and the parametersof the turbulent convective model (time-dependent mixing length) mainlyinfluence the modal selection and the period, which is then reflected in theperiod -- $\Delta \Phi_1$ diagram. We discuss the potential application of thisobservable as a discriminant for pulsation modes and as a test for ultra-lowamplitudes (ULA) pulsation.Comment: 11 pages, 8 figures, accepted for publication in ApJ, minor revisions in the text and figures, (black and white version available from 2nd author's website