## 4. The nonlinear double mode solutionIn Table 1 the parameters of the nonlinear double mode solution are summarised. A comparison with observed period and amplitude ratios of both, globular clusters and the four known double mode RR Lyrae stars in the Galactic field is given in Fig. 4. Concerning the amplitude ratios it is important to note that we compare theoretical bolometric ratios with observed ratios meassured in different photometric systems. In order to estimate the deviations we calculate a detailed frequency-dependent radiative transfer by solving the static radiative transfer equation on the dynamical structure of the model. The radiative opacities are taken from Kurucz (1993) and the computed amplitude ratios for the Johnson UBV system are given in Table 2. It turns out that the relative deviations are below 5% indicating that in contrast to the amplitudes the amplitude ratios are only weakely dependent on the photometric system.
Fig. 3A shows the bolometric light curve of the double mode model. The bolometric amplitude corresponds to and the luminosity variation over consecutive cycles presents the typical beat shape. The light curves phased according to the fundamental and first overtone periods are depicted in Fig. 3D and 3E, respectively. Fig. 3B refers to the fundamental mode light curve after removal of the first overtone and Fig. 3C to the same but for the first overtone, again phased according to the corresponding mode. We want to emphasize the bump close to the maximum light in Fig. 3C and E which is common both in observed and theoretical RRc light curves (e.g. Lub 1977, Feuchtinger & Dorfi 1994, Bono et al. 1997). The Fourier analysis of the light curve is listed in Table 3 and shows that only the fundamental and first overtone frequencies and corresponding linear combinations are present.
© European Southern Observatory (ESO) 1998 Online publication: August 17, 1998 |