On the α-element gradients of the Galactic thin disk using Cepheids^⋆,⋆⋆

¹ Dipartimento di Fisica, Università di Roma Tor Vergata, via della Ricerca Scientifica 1, 00133 Rome, Italy
e-mail: ron.oliveirasilva@gmail.com
² Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, PO Box 94249, 1090 GE Amsterdam, The Netherlands
³ INAF−Osservatorio Astronomico di Roma, via Frascati 33, Monte Porzio Catone, 00136 Rome, Italy
⁴ INAF−Osservatorio Astronomico di Teramo, via Mentore Maggini s.n.c., 64100 Teramo, Italy
⁵ Max-Planck-Institut für Astronomy, 69117 Heidelberg, Germany
⁶ Institute of Astronomy, University of Cambridge, Madingley Road, CB3 0HA, Cambridge, UK
⁷ GEPI, Observatoire de Paris, CNRS, Université Paris Diderot, Place Jules Janssen, 92190 Meudon, France
⁸ UPJV, Université de Picardie Jules Verne, 33 rue St. Leu, 80080 Amiens, France
⁹ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
¹⁰ Department of Physics and Astronomy, N283 ESC, Brigham Young University, Provo, UT 84601, USA
¹¹ South African Astronomical Observatory, PO Box 9, Observatory 7935, South Africa
¹² Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822, USA
¹³ Max-Planck-Institute for Astrophysics, Karl-Schwarzschild-Str.1, 85741 Garching, Germany
¹⁴ University Observatory Munich, Scheinerstr. 1, 81679 Munich, Germany
¹⁵ Department of Astronomy, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, 113-0033 Tokyo, Japan
¹⁶ INAF−Osservatorio Astronomico di Trieste, via G. B. Tiepolo 11, 34143 Trieste, Italy
¹⁷ Institute for Astro- and Particle Physics, University of Innsbruck, Technikerstr. 25/8, 6020 Innsbruck, Austria
¹⁸ Laboratoire Lagrange, CNRS/UMR 7293, Observatoire de la Côte d’Azur, Bd de l’Observatoire, CS 34229, 06304 Nice, France

Received: 13 February 2015
Accepted: 11 March 2015

Abstract

We present new homogeneous measurements of Na, Al, and three α-elements (Mg, Si, Ca) for 75 Galactic Cepheids. The abundances are based on high spectral resolution (R~ 38 000) and high signal-to-noise ratio (S/N ~ 50−300) spectra collected with UVES at ESO VLT. The current measurements were complemented with Cepheid abundances provided by our group (75) or available in the literature, for a total of 439 Galactic Cepheids. Special attention was given to providing a homogeneous abundance scale for these five elements plus iron. In addition, accurate Galactocentric distances (R_G) based on near-infrared photometry are also available for all the Cepheids in the sample. They cover a large section of the Galactic thin disk (4.1 ≤R_G≤ 18.4 kpc). We found that these five elements display well-defined linear radial gradients and modest standard deviations over the entire range of R_G. Moreover, the [element/Fe] abundance ratios are constant across the entire thin disk; only the Ca radial distribution shows marginal evidence of a positive slope. These results indicate that the chemical enrichment history of iron and of the quoted five elements has been quite similar across the four quadrants of the Galactic thin disk. The [element/Fe] ratios are also constant over the entire period range. This empirical evidence indicates that the chemical enrichment of Galactic Cepheids has also been very homogenous within the range in age that they cover (~10−300 Myr). Once again, [Ca/Fe] vs. log P shows a (negative) gradient, since it is underabundant among the youngest Cepheids. Finally, we also find that Cepheid abundances agree quite well with similar abundances for thin and thick disk dwarf stars, and they follow the typical Mg−Al and Na−O correlations.