Astron. Astrophys. 363, 537-554 (2000)

1. Introduction

The chemical evolution of the interstellar medium (ISM) varies between galaxies and is both position and time dependent within a galaxy. For example, metallicity studies of objects in the Magellanic Clouds (see Rolleston et al. 1993b, 1996) demonstrate that the global environments of these dwarf, irregular galaxies are completely different to that of the Milky Way. Furthermore, studies of young, B-type stars in Galactic clusters/associations and field FG-type dwarfs have shown the metal content of the ISM to differ by up to 0.5 dex over relatively small (1 kpc) spatial scales (see Rolleston et al. 1994; Edvardsson et al. 1993). Temporal variations have also been investigated and an age-metallicity relationship has been constructed using observations of the Galactic globular cluster and open cluster systems (see, for example, Strobel 1991; Carraro & Chiosi 1994). These variations are a consequence of successive generations of stars having enriched the ISM with products of nucleosynthesis that were generated in their cores. Thus, abundance investigations provide a key to understanding the formation and chemical evolution of galaxies.

However, theoretical models of the chemical and dynamical evolution of the Galaxy depend on many variables including the historical star formation rate (Phillipps & Edmunds 1991), the initial mass function (Güsten & Metzger 1982), radial inflows/outflows of gas (Mayor & Vigroux 1981) and the infall of metal-poor gas from the Galactic halo (Pilyugin & Edmunds 1996; van den Hoek & de Jong 1997). The radial variations of metallicity within the Galactic disk (ie. abundance gradients) predicted by such models are not tightly constrained. Thus, the reliable determination of the extent and magnitude of radial and spatial variations for a wide range of elements must be used to constrain models of disk evolution (see, for example, Matteucci & Francois 1989; Edmunds & Greenhow 1995; Prantzos & Aubert 1995, Portinari & Chiosi 1999).

Considerable effort has been invested in studying metallicity variations within our Galaxy and external galaxies. The existence of large-scale abundance gradients were first established by Searle (1971) in a survey of H II regions in six late-type spiral (Sc) galaxies (see also Garnett et al. 1997, for a recent review and comparison of abundance gradients in normal spiral galaxies). Early investigations of a metallicity gradient in the Galactic disk included the photometric studies of old open clusters (Janes 1979) and evolved disk stars (Mayor 1976). However, most information concerning radial abundance gradients have been derived from the spectroscopic investigation of H II regions (Pagel & Edmunds 1981; Shaver et al. 1983; Afflerbach et al. 1997) and planetary nebulae (Torres-Peimbert & Peimbert 1977; Maciel & Köppen 1994), as observations of these bright, gaseous nebulae are relatively easy to obtain. Such large-scale gradients now appear to be well established in both our Galaxy and in other spiral galaxies (see, for example, the recent reviews by Pagel 1997and McWilliam 1997). In Table 1, an overview is given of published estimates for the magnitudes of the abundance gradients. It should be noted that this compilation is not meant to be exhaustive, but rather a selection that is representative of that which can be found in the literature.

Table 1. A compilation of observed radial metallicity gradients in the Galactic disk, grouped by object type, is presented. Where more than one element is sampled, the value of the gradient is either the mean or a representative value of all the elements in that study. This list is not meant to be an exhaustive catalogue of individual gradients derived by each author for each element, but an illustration of work already undertaken. The more important comparisons are discussed in Sect. 6, and the reader is referred to the references below for details of each result.

However, relatively few elements have been studied in a homogeneous and consistent manner to date and the precise behaviour as a function of Galactocentric radius () is still rather uncertain. For example, Shaver et al. (1983) found radial abundance variations of approximately -0.07 dex kpc^-1 for oxygen, nitrogen and argon from a study of 67 H II regions within the range of Galactocentric distances, 4.0-12.5 kpc - with some evidence for a steepening of the gradient towards the inner region ( kpc). This contrasts with the results of the photometric study of intermediate age clusters by Janes (1979), which indicated that the gradient steepened in the outer Galactic disk (at  kpc). More recently, Twarog et al. (1997) have suggested that the metallicity distribution of clusters as a function of Galactocentric distance is best described by distinct zones (and a discontinuity), rather than a continuous linear decline in metallicity. Given that the metallicity studies of H II regions and intermediate-to-old clusters sample different epochs in the Galaxy's history, it is essential that one relates results from comparable objects when attempting to correlate the data.

© European Southern Observatory (ESO) 2000

Online publication: December 11, 2000
helpdesk.link@springer.de