Structural Properties of s-Cepheid Velocity Curves: Constraining the Location of the ω₄ = 2ω₁ Resonance

Abstract

The light curves of the first overtone Pop I Cepheids (s-Cepheids) show a discontinuity in their ϕ₂₁ vs. P diagram, near P = 3.2 d. This feature, commonly attributed to the 2:1 resonance between the first and the fourth overtones (ω₄ ≈ 2ω₁), is not reproduced by the hydrodynamical models (Antonello & Aikawa 1995). With the goal of reexamining the resonance hypothesis, we have obtained new CORAVEL and CORALIE radial velocity curves for 17 overtone Cepheids. Together with 10 objects from Krzyt et al. (2000), the combined sample covers the whole range of overtone Cepheid periods. The velocity Fourier parameters display a strong characteristic resonant behavior. In striking contrast to photometric ones, they vary smoothly with the pulsation period and show no jump at 3.2 d. The existing radiative hydrodynamical models match very well the velocity parameters. The center of the ω₄ = 2ω₁ resonance is estimated to occur at P_r = 4.58 ± 0.04 d, i.e. at a period considerably longer than previously assumed (3.2 d). Five new s-Cepheids, with periods above 5 d, have been identified from the radial velocity curves morphology: V419 Cen, V659 Cen, MY Pup, GH Car and V440 Per. The data and the discussion can be found in Kienzle et al. (1999). Preprints and the data are available at: http://obswww.unige.ch/˜kienzle.