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


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

D.D. Sasselov 1, 2, J.P.Beaulieu 2, C. Renault 3, P. Grison 2, R.Ferlet 2, A. Vidal-Madjar 2, E. Maurice 5, L. Prévot 5, E. Aubourg 3, P. Bareyre 3, S. Brehin 3, C. Coutures 3, N. Delabrouille 3, J. de Kat 3, M. Gros 3, B. Laurent 3, M. Lachièze-Rey 3, E. Lesquoy 3, C. Magneville 3, A. Milsztajn 3, L. Moscoso 3, F. Queinnec 3, J. Rich 3, M. Spiro 3, L. Vigroux 3, S. Zylberajch 3, R. Ansari 4, F. Cavalier 4, M. Moniez 4, C. Gry 6, J. Guibert 7, O. Moreau 7 and F. Tajhmady 7

1 Harvard-Smithsonian Center for Astrophysics, 60 Garden St., MA 02138, Cambridge, USA
2 Institut d'Astrophysique de Paris CNRS, 98bis boulevard Arago, F-74014 Paris France
3 CEA, DSM/DAPNIA, Centre d'études de Saclay, F-91191 Gif-sur-Yvette, France
4 Laboratoire de l'Accélérateur Linéaire IN2P3, Centre d'Orsay, F-91405 Orsay, France
5 Observatoire de Marseille, 2 place Le Verrier, F-13248 Marseille 04, France
6 Laboratoire d'Astronomie Spatiale CNRS, Traversée du siphon, les trois lucs, F-13120 Marseille, France
7 Centre d'Analyse des Images de l'Institut National des Sciences de l'Univers, CNRS Observatoire de Paris, 61 Avenue de l'Observatoire, F-75014 Paris, France

Received 12 November 1996 / Accepted 13 February 1997

Abstract

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 [FORMULA] & [FORMULA]) 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 [FORMULA] mag in the [FORMULA] 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 [FORMULA]. When applied to recent estimates based on HST Cepheid observations, our metallicity dependence makes the low- [FORMULA] values (Sandage et al. 1994) higher and the high- [FORMULA] values (Freedman et al. 1994b) lower, thus bringing those discrepant estimates into agreement near [FORMULA] km.s [FORMULA] [FORMULA].

Key words: stars: Cepheids – distance – stars: abundances

* This work is based on observations at the European Southern Observatory, La Silla, Chile.

Send offprint requests to: D. Sasselov (USA address)

© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998

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