Structural Properties of s-Cepheid Velocity Curves

TL;DR

This study reexamines the resonance hypothesis in s-Cepheid velocity curves, finding the resonance at a longer period than previously thought and identifying new s-Cepheid members, supported by new radial velocity data and hydrodynamical models.

Contribution

It provides new radial velocity measurements for overtone Cepheids and revises the resonance period, challenging previous assumptions and confirming hydrodynamical model predictions.

Findings

Resonance occurs at P_r = 4.58 days, longer than 3.2 days.

Velocity Fourier parameters show smooth variation with period.

Hydrodynamical models match observed velocity parameters well.

Abstract

The light curves of the first overtone Pop. I Cepheids (s-Cepheids) show a discontinuity in their phi_21 vs. P diagram, near P=3.2 day. This feature, commonly attributed to the 2:1 resonance between the first and the fourth overtones (omega_4=2 omega_1), is not reproduced by the hydrodynamical models. With the goal of reexamining the resonance hypothesis, we have obtained new CORAVEL radial velocity curves for 14 overtone Cepheids. Together with 10 objects of Krzyt et al. (1999), 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 day. The existing radiative hydrodynamical models match the velocity parameters very well. The center of the omega_4 = 2 omega_1 resonance is estimated to occur at P_r = 4.58+-0.04 day, i.e. at a period considerably longer than previously assumed (3.2 day). We identify two new members of the s-Cepheid group: MY Pup and V440 Per.