On the pulsation mode identification of short-period Galactic Cepheids

We present new theoretical period-radius relations for first-overtone Galactic Cepheids. Current predictions are based on several sequences of nonlinear, convective pulsation models at solar chemical composition (Y = 0.28, Z = 0.02) and stellar masses ranging from 3.0 to 5.5 M.. The comparison between predicted and empirical radii of four short-period Galactic Cepheids suggests that QZ Normae and EV Scuti are pulsating in the fundamental mode, whereas Polaris and SZ Tauri pulsate in the first overtone. This finding supports the mode identifications that rely on the comparison between direct and period-luminosity-based distance determinations, but it is somewhat at variance with the mode identification based on Fourier parameters. In fact, we find from our models that fundamental and first-overtone pulsators attain, for periods ranging from 2.7 to 4 days, quite similar phi (21)-values, making mode discrimination from this parameter difficult. The present mode identifications for our sample of Cepheids are strengthened by the accuracy of their empirical radius estimates as well as by the evidence that predicted fundamental and first-overtone radii do not show, within the current uncertainty on the mass-luminosity relation, any degeneracy in the same period range. Accurate radius determinations are therefore an excellent tool to unambiguously determine the pulsation modes of short-period Cepheids.