On the fine structure of the Cepheid metallicity gradient in the Galactic thin disk ^⋆,⋆⋆

¹ Dipartimento di Fisica, Università di Roma Tor Vergata, via della Ricerca Scientifica 1, 00133 Rome, Italy
e-mail: katia.genovali@roma2.infn.it
² Astronomical Institute Anton Pannekoek, Science Park 904, PO Box 94249, 1090 GE Amsterdam, The Netherlands
³ INAF – Osservatorio Astronomico di Roma, via Frascati 33, Monte Porzio Catone, Rome, Italy
⁴ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei Munchen, Germany
⁵ INAF – Osservatorio Astronomico di Collurania, via M. Maggini, 64100 Teramo, Italy
⁶ Department of Physics and Astronomy, N283 ESC, Brigham Young University, Provo, UT 84601, USA
⁷ South African Astronomical Observatory, PO Box 9, Observatory 7935, South Africa
⁸ INAF – Osservatorio Astronomico di Trieste, via G.B. Tiepolo 11, 40131 Trieste, Italy
⁹ Max-Planck-Institut fur Astrophysik, Karl-Schwarzschild-Str. 1, 85741 Garching, Germany
¹⁰ Institute of Astronomy, University of Cambridge, Madingley Road, CB3 0HA, Cambridge, UK
¹¹ GEPI – Observatoire de Paris, 64 avenue de l’Observatoire, 75014 Paris, France
¹² UPJV – Université de Picardie Jules Verne, 80000 Amiens, France
¹³ Institute for Astronomy, University of Hawai’i, 2680 Woodlawn Dr, Honolulu, HI 96822, USA
¹⁴ University Observatory Munich, Scheinerstr. 1, 81679 Munich, Germany
¹⁵ Kiso Observatory, Institute of Astronomy, School of Science, The University of Tokyo 10762-30, Mitake, Kiso-machi, Kiso-gun, 97-0101 3 Nagano, Japan
¹⁶ Laboratoire Lagrange, UMR 7293, Université de Nice Sophia-Antipolis, CNRS, Observatoire de la Côte d’Azur, 06300 Nice, France

Received: 5 December 2013
Accepted: 19 March 2014

Abstract

We present homogeneous and accurate iron abundances for 42 Galactic Cepheids based on high resolution (R ~ 38 000) high signal-to-noise ratio (S/N ≥ 100) optical spectra collected with UVES at VLT (128 spectra). The above abundances were complemented with high-quality iron abundances provided either by our group (86) or available in the literature. We were careful to derive a common metallicity scale and ended up with a sample of 450 Cepheids. We also estimated accurate individual distances for the entire sample by using homogeneous near-infrared photometry and the reddening free period-Wesenheit relations. The new metallicity gradient is linear over a broad range of Galactocentric distances (R_G ~ 5–19 kpc) and agrees quite well with similar estimates available in the literature (–0.060 ± 0.002 dex/kpc). We also uncover evidence that suggests that the residuals of the metallicity gradient are tightly correlated with candidate Cepheid groups (CGs). The candidate CGs have been identified as spatial overdensities of Cepheids located across the thin disk. They account for a significant fraction of the residual fluctuations, and also for the large intrinsic dispersion of the metallicity gradient. We performed a detailed comparison with metallicity gradients based on different tracers: OB stars and open clusters. We found very similar metallicity gradients for ages younger than 3 Gyr, while for older ages we found a shallower slope and an increase in the intrinsic spread. The above findings rely on homogeneous age, metallicity, and distance scales. Finally, by using a large sample of Galactic and Magellanic Cepheids for which accurate iron abundances are available, we found that the dependence of the luminosity amplitude on metallicity is vanishing.