On the pulsation mode identification of short-period Galactic Cepheids

We present new theoretical period-radius relations for first-overtone Galac tic Cepheids. Current predictions are based on several sequences of nonline ar, 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 betwe en predicted and empirical radii of four short-period Galactic Cepheids sug gests that QZ Normae and EV Scuti are pulsating in the fundamental mode, wh ereas Polaris and SZ Tauri pulsate in the first overtone. This finding supp orts the mode identifications that rely on the comparison between direct an d period-luminosity-based distance determinations, but it is somewhat at va riance with the mode identification based on Fourier parameters. In fact, w e 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, ma king mode discrimination from this parameter difficult. The present mode id entifications for our sample of Cepheids are strengthened by the accuracy o f their empirical radius estimates as well as by the evidence that predicte d fundamental and first-overtone radii do not show, within the current unce rtainty on the mass-luminosity relation, any degeneracy in the same period range. Accurate radius determinations are therefore an excellent tool to un ambiguously determine the pulsation modes of short-period Cepheids.