Up to now no complete and detailed study of nonlinear models has been published. Preliminary results for purely radiative models for Z values of 0.020, 0.015 and 0.010 were reported by Buchler (1998): the values for P near 10 d depend strongly on Z, in the sense that the spread of the values decreases with decreasing Z. That is, for models with , the values vary in a range between 4 and 5.5 rad, while for vary between 4.5 and 4.8 rad. This trend is confirmed by the radiative models of Aikawa & Antonello (2000), that for show very uniform values. However, the Fourier parameters and of light curves of Cepheids with d in Galaxy are similar to those of Cepheids with the same P in LMC () and SMC (), and do not support the theoretical model predictions; for LMC and SMC, see the results of MACHO (e.g. Welch et al., 1997), EROS (e.g. Beaulieu & Sasselov 1997) and OGLE (e.g. Udalski et al. 1999a) surveys. IC 1613 Cepheids could be compatible with these model predictions, but the average Z value, 0.001, is much lower; in other words, it would seem that the models are too much sensitive to metallicity differences. As regards the models which include convection, the situation does not look much better (J.R. Buchler, private communication); for example, the light curves obtained by Bono et al. (1999) look rather different from those shown in Fig. 2.
In order to verify possible biases, we made a comparison with SMC Cepheids using OGLE database (Udalski et al. 1999b). Most of OGLE observations were performed in I band, whose effective wavelength is quite different from that of V and Wh bands; therefore we considered the V datasets for about 60 stars in the P range between 7 and 11 d, even if the number of data points per set were not large (about 30). An inspection of the V light curves and of the respective Fourier parameters showed that the characteristics of some of SMC Cepheids are not very different from those of IC 1613 with similar period, and therefore the apparent partial compatibility with the theoretical model predictions could be due just to the poor number of Cepheids in IC 1613. In particular, we remark that the observed spread of values at d is explained mainly by stars with a light curve shape similar to that of FN Aql, that is a light curve which is rather symmetric or with a small bump on the ascending branch. This kind of stars was not detected in IC 1613.
© European Southern Observatory (ESO) 2000
Online publication: April 10, 2000