Metallicity dependence of the Cepheid calibration

The metallicity dependence of the Cepheid period-luminosity (PL) relation has been a controversial issue. The more commonly accepted view is that the PL relation is insensitive to metal abundance, while the period-luminosity-colour relation is fairly sensitive, as indicated by stellar pulsation models1−4 (see also refs. 5,6). The direct observational evidence is scanty, however. Freedman and Madore7 have studied Cepheids in three regions having different metallicity in M31 and concluded no statistically significant residual differences in the distance moduli. On the other hand, Gould8 has claimed that the data of Freedman and Madore are rather indicating a large metallicity dependence ∆µ/∆[Fe/H] = 0.88±0.16, where µ is the distance modulus derived from the Cepheid PL relation. A moderate metallicity dependence ∆µ/∆[Fe/H] = 0.44 was also concluded in a recent report from EROS photometry for the Magellanic Clouds9. Recently an accurate measurement of metallicity has been carried out for 23 Galactic Cepheids by Fry and Carney10 (hereafter FC). This work has given us an opportunity to examine the metallicity dependence of the Cepheid PL relation and its calibrations. The most commonly used calibration of the Galactic Cepheids is probably that of Feast and Walker11 based on V band photometry and the distance based on the ZAMS fitting to the clusters that contain the Cepheids. Laney and Stobie12 (hereafter LS) have slightly updated the Feast-Walker distance and also provided a calibration of the Cepheid PL 1 relation for the near infrared JHK colour bands13 in addition to that for the V band (their result for V band hardly differs from that of Feast and Walker). The work of LS may perhaps be taken as the most accurate calibration of the Cepheid PL relation to date, hence giving the most accurate distance to the LMC with the Cepheid method. The FC sample contains 12 Cepheids in common with the LS sample: EV Sct, V340