The effect of metallicity on Cepheid period-luminosity relations from a Baade-Wesselink analysis of Cepheids in the Milky Way and Magellanic Clouds^⋆,⋆⋆

¹ Universidad de Concepción, Departamento Astronomía, Casilla 160-C, Concepción, Chile
e-mail: wgieren@astro-udec.cl
² Millenium Institute of Astrophysics, Santiago, Chile
³ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
⁴ Obserwatorium Astronomiczne, Uniwersytet Warszawski, Al. Ujazdowskie 4, 00-478 Warsaw, Poland
⁵ Centrum Astronomiczne im. Mikołaja Kopernika (CAMK), PAN, Bartycka 18, 00-716 Warsaw, Poland
⁶ Carnegie Observatories, 813 Santa Barbara Street, Pasadena, CA, 911101-1292 USA
⁷ University of Texas at Austin, McDonald Observatory, 1 University Station, C1402, Austin, TX, 78712-0259 USA
⁸ IRAP, Université de Toulouse, CNRS, 14 av. E. Belin, 31400 Toulouse, France
⁹ CFHT Corporation, 65-1238 Mamalahoa Hwy, Kamuela, HI, 96743 USA
¹⁰ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, France
¹¹ European Southern Observatory, Alonso de Córdova 3107, Casilla, 19001 Santiago, Chile

Received: 19 April 2018
Accepted: 7 September 2018

Abstract

Context. The extragalactic distance scale builds on the Cepheid period-luminosity (PL) relation. Decades of work have not yet convincingly established the sensitivity of the PL relation to metallicity. This currently prevents a determination of the Hubble constant accurate to 1% from the classical Cepheid-SN Ia method.

Aims. In this paper we carry out a strictly differential comparison of the absolute PL relations obeyed by classical Cepheids in the Milky Way (MW), LMC, and SMC galaxies. Taking advantage of the substantial metallicity difference among the Cepheid populations in these three galaxies, we want to establish a possible systematic trend of the PL relation absolute zero point as a function of metallicity, and to determine the size of such an effect in the optical and near-infrared photometric bands.

Methods. We used a IRSB Baade-Wesselink-type method to determine individual distances to the Cepheids in our samples in the MW, LMC, and SMC. For our analysis, we used a greatly enhanced sample of Cepheids in the SMC (31 stars) compared to the small sample (5 stars) available in our previous work. We used the distances to determine absolute Cepheid PL relations in the optical and near-infrared bands in each of the three galaxies.

Results. Our distance analysis of 31 SMC Cepheids with periods of 4–69 days yields tight PL relations in all studied bands, with slopes consistent with the corresponding LMC and MW relations. Adopting the very accurately determined LMC slopes for the optical and near-infrared bands, we determine the zero point offsets between the corresponding absolute PL relations in the three galaxies.

Conclusions. We find that in all bands the metal-poor SMC Cepheids are intrinsically fainter than their more metal-rich counterparts in the LMC and MW. In the K band the metallicity effect is −0.23 ± 0.06 mag dex⁻¹, while in the V, (V − I) Wesenheit index it is slightly stronger, −0.34 ± 0.06 mag dex⁻¹. We find suggestive evidence that the metallicity sensitivity of the PL relation might be nonlinear, being small in the range between solar and LMC Cepheid metallicity, and becoming steeper towards the lower-metallicity regime.