Galactic abundance gradients from Cepheids

α and heavy elements in the outer disk^⋆,⋆⋆

¹ Astronomical Institute Anton Pannekoek, Science Park 904, PO Box 94249, 1090 GE Amsterdam, The Netherlands
e-mail: B.J.P.Lemasle@uva.nl
² Kapteyn Astronomical Institute, Landleven 12, 9747 AD Groningen, The Netherlands
³ GEPI – Observatoire de Paris, 64 avenue de l’Observatoire, 75014 Paris, France
⁴ UPJV – Université de Picardie Jules Verne, 80000 Amiens, France
⁵ Dipartimento di Fisica, Università di Roma Tor Vergata, via della Ricerca Scientifica 1, 00133 Rome, Italy
⁶ Astronomical Observatory of Odessa National University, and Isaac Newton Institute of Chile, Odessa branch, Shevchenko Park, 65014 Odessa, Ukraine
⁷ INAF – Osservatorio Astronomico di Roma, via Frascati 33, Monte Porzio Catone, Rome, Italy
⁸ South African Astronomical Observatory, PO Box 9, Observatory 7935, South Africa
⁹ Department of Physics and Astronomy, Western Kentucky University, 1906 College Heights Blvd, Bowling Green, KY 42101-1077, USA
¹⁰ Max Planck Institute for Astrophysics, Karl-Schwarzschild-Str. 1, 85741 Garching bei Munchen, Germany
¹¹ INAF – Osservatorio Astronomico Collurania, via M. Maggini, 64100 Teramo, Italy
¹² Department of Astronomy, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, 113-0033 Tokyo, Japan
¹³ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei Munchen, Germany

Received: 21 June 2013
Accepted: 28 July 2013

Abstract

Context. Galactic abundance gradients set strong constraints to chemo-dynamical evolutionary models of the Milky Way. Given the period-luminosity relations that provide accurate distances and the large number of spectral lines, Cepheids are excellent tracers of the present-day abundance gradients.

Aims. We want to measure the Galactic abundance gradient of several chemical elements. While the slope of the Cepheid iron gradient did not vary much from the very first studies, the gradients of the other elements are not that well constrained. In this paper we focus on the inner and outer regions of the Galactic thin disk.

Methods. We use high-resolution spectra (FEROS, ESPADONS, NARVAL) to measure the abundances of several light (Na, Al), α (Mg, Si, S, Ca), and heavy elements (Y, Zr, La, Ce, Nd, Eu) in a sample of 65 Milky Way Cepheids. Combining these results with accurate distances from period-Wesenheit relations in the near-infrared enables us to determine the abundance gradients in the Milky Way.

Results. Our results are in good agreement with previous studies on either Cepheids or other tracers. In particular, we confirm an upward shift of ≈0.2 dex for the Mg abundances, as has recently been reported. We also confirm the existence of a gradient for all the heavy elements studied in the context of a local thermodynamic equilibrium analysis. However, for Y, Nd, and especially La, we find lower abundances for Cepheids in the outer disk than reported in previous studies, leading to steeper gradients. This effect can be explained by the differences in the line lists used by different groups.

Conclusions. Our data do not support a flattening of the gradients in the outer disk, in agreement with recent Cepheid studies and chemo-dynamical simulations. This is in contrast to the open cluster observations but remains compatible with a picture where the transition zone between the inner disk and the outer disk would move outward with time.