Using Cepheids to determine the galactic abundance gradient^*,**

I. The solar neighbourhood

¹ Instituto Astronômico e Geofísico, Universidade de São Paulo, Av. Miguel Stefano, 4200 São Paulo SP, Brazil e-mail: sergei@andromeda.iagusp.usp.br
² Department of Astronomy, Odessa State University, Shevchenko Park, 65014, Odessa, Ukraine e-mail: val@deneb.odessa.ua
³ Odessa Astronomical Observatory and Isaac Newton Institute of Chile, Odessa Branch, Ukraine
⁴ Department of Astronomy, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, OH 44106-7215, USA, e-mail: luck@fafnir.astr.cwru.edu
⁵ Visiting Astronomer, Cerro Tololo Inter-American Observatory, National Optical Astronomy Observatories which are operated by the Association of Universities for Research in Astronomy, Inc., under contract with the US National Science Foundation
⁶ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MS 16, Cambridge, MA 02138, USA e-mail: dbersier@cfa.harvard.edu
⁷ Special Astrophysical Observatory, Russian Academy of Sciences, Nizhny Arkhyz, Stavropol Territory, 369167, Russia e-mail: valenta@sao.ru; panchuk@sao.ru
⁸ SAO RAS and Isaac Newton Institute of Chile, SAO RAS Branch, Russia
⁹ EdIC group, Universidade de São Paulo, São Paulo, Brazil e-mail: ueda@ime.usp.be

Corresponding author: S. M. Andrievsky, scan@deneb.odessa.ua

Received: 31 July 2001
Accepted: 10 October 2001

Abstract

A number of studies of abundance gradients in the galactic disk have been performed in recent years. The results obtained are rather disparate: from no detectable gradient to a rather significant slope of about -0.1 dex kpc^-1. The present study concerns the abundance gradient based on the spectroscopic analysis of a sample of classical Cepheids. These stars enable one to obtain reliable abundances of a variety of chemical elements. Additionally, they have well determined distances which allow an accurate determination of abundance distributions in the galactic disc. Using 236 high resolution spectra of 77 galactic Cepheids, the radial elemental distribution in the galactic disc between galactocentric distances in the range 6–11 kpc has been investigated. Gradients for 25 chemical elements (from carbon to gadolinium) are derived. The following results were obtained in this study. Almost all investigated elements show rather flat abundance distributions in the middle part of galactic disc. Typical values for iron-group elements lie within an interval from -0.02 to -0.04 dex kpc^-1 (in particular, for iron we obtained d[Fe/H]/d dex kpc^-1). Similar gradients were also obtained for O, Mg, Al, Si, and Ca. For sulphur we have found a steeper gradient (-0.05 dex kpc^-1). For elements from Zr to Gd we obtained (within the error bars) a near to zero gradient value. This result is reported for the first time. Those elements whose abundance is not expected to be altered during the early stellar evolution (e.g. the iron-group elements) show at the solar galactocentric distance [El/H] values which are essentially solar. Therefore, there is no apparent reason to consider our Sun as a metal-rich star. The gradient values obtained in the present study indicate that the radial abundance distribution within 6–11 kpc is quite homogeneous, and this result favors a galactic model including a bar structure which may induce radial flows in the disc, and thus may be responsible for abundance homogenization.