Astron. Astrophys. 324, 471-482 (1997)

9. Conclusions

We use the new EROS microlensing survey data-set of 3 million two-color observations of about 500 Cepheids  in the LMC and SMC to search for and derive the dependence of the optical PL relations on metallicity. We find that:
(1) The PL relations for both types of Cepheids have the same zero-point offset (but no slope difference), which is attributed to the effect of metallicity under a reasonable assumption about the extinction law. As expected from theory, this effect of metal content is manifested in a color shift of the instability strip. It amounts to about of the strip width, and we could detect it unambiguously thanks to the large number of Cepheids  and extremely well sampled light curves.
(2) With the known ensemble difference in metal content between LMC and SMC Cepheids , we derive a linear relation between the distance modulus correction and metallicity:

It applies to distances which are inferred by using LMC as a base and using two color VI photometry of the Cepheids to establish the reddening. The linearity of metallicity dependence is a good assumption, but needs to be confirmed empirically outside the range of application (a factor of few lower than SMC and higher than the Galaxy).
(3) The first overtone Cepheids  have PL relations which provide distances fully consistent with the PL relations of fundamental mode Cepheids .

We use two color bands closely spaced in wavelength because of availability, not their desirability; ideally one would like to use at least three bands, one of them in the near-infrared.

Our result can be applied to the long-standing discrepancy between the low- scale and the high- scale. The host galaxies on which each of these scales relies appear to have systematically different metallicities. A simple application of our correction to several recent derivations makes the low- values (Sandage et al. 1994) higher and the high- values (Freedman et al. 1994b) lower, thus bringing those discrepant estimates into agreement near km.s .

© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998

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