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Astron. Astrophys. 354, 610-620 (2000)

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1. Introduction

Since the Leavitt (1912) discovery of the relationship between period and apparent magnitude for Cepheids in the Magellanic Clouds, these variables have been playing a fundamental role in the determination of distances to Local Group galaxies, in the calibration of various secondary distance indicators and finally in the evaluation of the Hubble constant [FORMULA]. A considerable amount of observational studies have been devoted to the calibration of their characteristic Period-Luminosity (PL) and Period-Luminosity-Color (PLC) relations, e.g. via the Baade-Wesselink method (Ripepi et al. 1997; Gieren et al. 1998 and references therein), or making use of Cepheids with distances from Galactic open cluster main-sequence fitting (Turner et al. 1998 and references therein), or adopting the Large Magellanic Cloud (LMC) distance found in other independent ways (see Walker 1999).

The most important question to be answered is whether the bolometric PL and PLC are universal , as early suggested by Iben & Renzini (1984), or depending on the chemical abundances of the variables. This is a fundamental point since if there is a metallicity effect on the relations in the various photometric bands and the galaxies whose distances we are deriving have metallicities different from the calibrating Cepheids, then a significant correction could be necessary. In recent times, the occurrence of a metallicity effect has become more and more evident, with the observational tests of this effect suggesting that metal-rich Cepheids are brighter than metal-poor Cepheids at fixed period (see Kochanek 1997; Beaulieu et al. 1997; Sasselov et al. 1997; Kennicutt et al. 1998). As for theoretical estimates, models based on linear pulsation calculations suggest a very low effect (Chiosi et al. 1993; Saio & Gautschy 1998; Alibert et al. 1999), whereas nonlinear, nonlocal and time-dependent convective pulsating models suggest that both the zero point and the slope of the predicted PL relations depend on metallicity, with the amplitude of the metallicity effect decreasing at the longer wavelengths (Bono et al. 1999b [Paper II]). Moreover, these models show that metal-rich Cepheids are on average intrinsically fainter than the metal-poor ones, at fixed period.

It is worth noticing that all the observational efforts rely on the assumption that the slope of the PL relation for different passbands is that found for LMC Cepheids, leading us to suspect that forcing the slope of the multiwavelength relations to be constant may introduce some systematic errors in the attempt of disentangling reddening from metallicity effects. On the other hand, at variance with the linear-nonadiabatic approach which supplies only the blue edge of the instability strip  1, the models presented in Paper II provide fine constraints on both blue and red limits of the pulsation, thus avoiding dangerous ad hoc assumptions on the width of the instability strip (see also Tanvir 1999). These models give also the pulsation amplitude and the predicted mean magnitudes of the pulsator.

The theoretical PL and PLC relations presented in Paper II deal with the intensity-weighted [FORMULA], [FORMULA] and [FORMULA] magnitudes. In order to have a full insight into the metallicity effect, and considering that the HST observations of extragalactic Cepheids consider the I band, in this paper we present the full set of theoretical relations (i.e. Period-Luminosity, Period-Color, Color-Color and Period-Luminosity-Color) in the BVRIJK bands. The predicted relations are reported in Sect. 2, while Sect. 3 gives brief comparisons of our theoretical results against observed data for calibrating Cepheids. Some final remarks close the paper.

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© European Southern Observatory (ESO) 2000

Online publication: February 9, 2000
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