EMPIRICAL APPROACH TO CEPHEID PERIOD-LUMINOSITY-COLOR AND PERIOD-LUMINOSITY RELATIONS BY SURFACE BRIGHTNESS

摘要

Empirical period-luminosity-color and period-luminosity (PLC[V, B - V] and PL[V] relations according to Stefan's law are derived for Cepheid variables by applying thermal scales of nonvariable giant and supergiant stars established by modern observational interferometry. Intrinsic surface brightnesses are predicted by the apparent color index V - K as fiducial reference unbiased by metallicity and gravity effects, along with current optical absorption data. Linear radii are inferred from periods through the period-radius relation, assumed to be insensitive to abundance and brightness variations. The dependence of PLC and PL zero points to abundance effects is further investigated. I compare current Cepheid data from our metal-normal Galaxy (GAL) with those in the Magellanic Clouds (LMC and SMC) of poorer metal content. The major achievement is that the PL(V) relations are found to be severely biased by metallicity, whereas the PLC(V, B - V) relations turn out to be much less affected by abundances than previously expected. The new PLC and PL relations calibrated on fundamental quantities are compared with those from the classical zero-age main-sequence (ZAMS) fitting empirical approach. It is shown that all metal-normal equations lead to remarkably consistent luminosities of GAL Cepheids, to within ± 0.05 mag (PL) and ± 0.02 mag (PLC), whereas metal-poor LMC and SMC results appear to be significantly discrepant. It is emphasized that theoretical thermal scales applied for the derivation of ZAMS metal-corrected relations, either explicit or implicit, are mainly responsible for such observed discrepancies. The actual empirical approach appears to overcome most of the previous difficulties closely related to the poor knowledge of Cepheid thermal properties. Theoretical Cepheid thermal scales are critically reviewed. It is found that recent metal-normal colorbrightness relations lead to results remarkably consistent with those from observational scales, whereas metalpoor predictions appear to be overestimated, at least for Cepheid periods log P (days) > 1 currently observed in extragalactic work. Theoretical PLC(V, B - V) and PL(V) relations revised according to unbiased conversion scales now support the above finding that PLC relations are less affected by metallicity than PL relations. This also enables the reconciliation of evolution, pulsation, and stellar atmosphere theories since the ratio of evolution to pulsation mass is found to be very close to 1 for all GAL, LMC, and SMC Cepheids. Because of the poorer metal content of the Magellanic Clouds, current PL(V) distance to the LMC and SMC are found to be underestimated, being referred to metal-normal GAL Cepheids as primary calibrators. Distance moduli through either infrared photometry (V, V - K) (pulsation parallaxes) or optical photometry (V, B - V) now become (m - M)_0(LMC) = 18.64 ± 0.02, (m - M)_0(SMC) = 19.09 ± 0.02 .