On the metallicity distribution of classical Cepheids in the Galactic inner 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 GEPI − Observatoire de Paris, 64 Avenue de l’Observatoire, 75014 Paris, France
⁷ UPJV − Université de Picardie Jules Verne, 80000 Amiens, France
⁸ Department of Physics and Astronomy, N283 ESC, Brigham Young University, Provo, UT 84601, USA
⁹ South African Astronomical Observatory, PO Box 9, Observatory 7935, South Africa
¹⁰ Kiso Observatory, Institute of Astronomy, School of Science, The University of Tokyo 10762-30, Mitake, Kiso-machi, Kiso-gun 3, 97-0101 Nagano, Japan
¹¹ Laboratoire Lagrange, UMR 7293, Université de Nice Sophia-Antipolis, CNRS, Observatoire de la Côte d’Azur, 06300 Nice, France

Received: 5 April 2013
Accepted: 22 April 2013

Abstract

We present homogeneous and accurate iron abundances for almost four dozen (47) of Galactic Cepheids using high-spectral resolution (R ~ 40 000) high signal-to-noise ratio (S/N ≥ 100) optical spectra collected with UVES at VLT. A significant fraction of the sample (32) is located in the inner disk (R_G ≤ 6.9 kpc) and for half of them we provide new iron abundances. Current findings indicate a steady increase in iron abundance when approaching the innermost regions of the thin disk. The metallicity is super-solar and ranges from 0.2 dex for R_G ~ 6.5 kpc to 0.4 dex for R_G ~ 5.5 kpc. Moreover, we do not find evidence of correlation between iron abundance and distance from the Galactic plane. We collected similar data available in the literature and ended up with a sample of 420 Cepheids. Current data suggest that the mean metallicity and the metallicity dispersion in the four quadrants of the Galactic disk attain similar values. The first-second quadrants show a more extended metal-poor tail, while the third-fourth quadrants show a more extended metal-rich tail, but the bulk of the sample is at solar iron abundance. Finally, we found a significant difference between the iron abundance of Cepheids located close to the edge of the inner disk ([Fe/H]  ~ 0.4) and young stars located either along the Galactic bar or in the nuclear bulge ([Fe/H]  ~ 0). Thus suggesting that the above regions have had different chemical enrichment histories. The same outcome applies to the metallicity gradient of the Galactic bulge, since mounting empirical evidence indicates that the mean metallicity increases when moving from the outer to the inner bulge regions.