Metallicity effects on the Cepheid extragalactic distance scale from EROS photometry in the Large Magellanic Cloud and the Small Magellanic Cloud.

Details Metallicity effects on the Cepheid extragalactic distance scale from EROS photometry in the Large Magellanic Cloud and the Small Magellanic Cloud. Metallicity effects on the Cepheid extragalactic distance scale from EROS photometry in the Large Magellanic Cloud and the Small Magellanic Cloud.

Aug 1997

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997a%26a...324..471s&link_type=abstract

Astronomy and Astrophysics, v.324, p.471-482

Astronomy and Astrophysics

Astrophysics

138

Stars: Cepheids, Distance, Stars: Abundances

Scientific paper

This is an investigation of the period-luminosity relation of classical Cepheids in samples of different metallicity. It is based on 481 Cepheids in the Large and Small Magellanic Clouds from the blue and red filter CCD observations (most similar to V_J_ & R_J_) of the French EROS microlensing project. The data-set is complete and provides an excellent basis for a differential analysis between LMC and SMC. In comparison to previous studies of effects on the PL-relation, the EROS data-set offers extremely well-sampled light curves and well-filled instability strips. This allows reliable separation of Cepheids pulsating in the fundamental and the first overtone mode and derivation of differential reddening. Our main result concerns the determination of distances to galaxies which are inferred by using the LMC as a base and using two color photometry to establish the amount of reddening. We find a zero-point offset between SMC and LMC which amounts to a difference between inferred and true distance modulus of 0.14+/-0.06mag in the VI_c_ system. The offset is exactly the same in both sets of PL-relations - of the fundamental and of the first overtone mode Cepheids. No effect is seen on the slopes of the PL-relations, although the fundamental and the first overtone mode Cepheids have different PL slopes. We attribute the color and the zero-point offset to the difference in metallicity between the SMC and LMC Cepheids. A metallicity effect of that small magnitude still has important consequencies for the inferred Cepheid distances and the determination of H_0_. When applied to recent estimates based on HST Cepheid observations, our metallicity dependence makes the low-H_0_ values (Sandage et al. 1994) higher and the high-H_0_ values (Freedman et al. 1994b) lower, thus bringing those discrepant estimates into agreement near H_0_~70 km/s/Mpc.

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