1 Introduction

The chemical abundances of long-lived F and G main sequence stars, combined with kinematical data and ages, provide a powerful way to probe the chemical and dynamical evolution of the Galaxy.

As far as the disk stars are concerned, many general trends have been discovered during the past decades. Most notable results are correlations of metallicity with age, Galactocentric distance, and vertical distance from the Galactic plane based on photometric or low-resolution observations (e.g. Eggen et al. [1962]; Twarog [1980]). In addition, the abundance patterns for some elements have been derived for small samples of stars: oxygen and $\alpha$ elements relative to iron vary systematically from overabundances at $\mbox{\rm [Fe/H]}\simeq -1.0$ to a solar ratio at $\mbox{\rm [Fe/H]}\simeq 0.0$, while most iron-peak elements follow iron for the whole metallicity range of the disk. These results have provided important constraints on chemical evolution models for the Galactic disk.

With improved observation and analysis techniques, which make it possible to study the Galactic chemical evolution (GCE) in detail, some old conclusions have, however, been challenged and new questions have arisen. Particularly important is the detailed abundance analysis of 189 F and G dwarfs with $-1.1<\mbox{\rm [Fe/H]}<0.25$ by Edvardsson et al. ([1993a], hereafter EAGLNT). The main results from this work may be summarized as follows: (1) There are no tight relations between age, metallicity and kinematics of disk stars, but substantial dispersions imposed on weak statistical trends. (2) There exists a real scatter in the run of [$\alpha$/Fe] vs. [Fe/H] possibly due to the mixture of stars with different origins. The scatter seems to increase with decreasing metallicity starting at $\mbox{\rm [Fe/H]}\simeq -0.4$. Together with a possible increase in the dispersion of W_LSR (the stellar velocity perpendicular to the Galactic plane with respect to the Local Standard of Rest, LSR) at this point, the result suggests a dual model for disk formation. It is, however, unclear if the transition at $\mbox{\rm [Fe/H]}\simeq -0.4$ represents the division between the thin disk and the thick disk. (3) A group of metal-poor disk stars with $\mbox{$R_{\rm m}$ }< 7$ kpc is found to have larger [$\alpha$/Fe] values than stars with $\mbox{$R_{\rm m}$ }> 9$ kpc, indicating a higher star formation rate (SFR) in the inner disk than that in the outer disk. Since essentially all the oldest stars in EAGLNT have small $\mbox{$R_{\rm m}$ }$, it is, however, difficult to know upon which, $\mbox{$R_{\rm m}$ }$ or age, the main dependence of [$\alpha$/Fe] is. (4) At a given age and $\mbox{$R_{\rm m}$ }$, the scatter in [$\alpha$/Fe] is negligible while [Fe/H] does show a significant scatter. The former implies that the products of supernovae of different types are thoroughly mixed into the interstellar medium (ISM) before significant star formation occurs. Based on this, the large scatter in [Fe/H] may be explained by infall of unprocessed gas with a characteristic mixing time much longer than that of the gas from supernovae of different types. (5) The Galactic scatter may be different for individual $\alpha$elements; [Mg/Fe] and [Ti/Fe] show a larger scatter at a given metallicity than [Si/Fe] and [Ca/Fe]. It suggests that individual $\alpha$ elements may have different origins. (6) A new stellar group, rich in Na, Mg, Al, was found among the metal-rich disk stars, suggesting additional synthesis sources for these elements.

Given that the study of EAGLNT was based on a limited sample of stars with certain selection effects and that the analysis technique induced uncertainties in the final abundances, some subtle results need further investigation before they can provide reliable constraints on theory. For example, it is somewhat unclear if the different [$\alpha$/Fe] at a given metallicity between the inner disk and the outer disk stars is real and if old disk stars are always located in the inner disk. Moreover, recent work by Tomkin et al. ([1997]) argued against the existence of NaMgAl stars. In addition, a number of elements, which are highly interesting from a nucleosynthetic point of view, were not included in the work of EAGLNT.

The present work, based on a large differently selected sample of disk stars, aims at exploring and extending the results of EAGLNT with improved analysis techniques. Firstly, we now have more reliable atmospheric parameters. The effective temperature is derived from the Strömgren b-y color index using a recent infrared-flux calibration and the surface gravity is based on the Hipparcos parallax. About one hundred iron lines (instead of $\sim 30$in EAGLNT) are used to provide better determinations of metallicity and microturbulence. Secondly, the abundance calculation is anchored at the most reliable theoretical or experimental oscillator strengths presently available in the literature. Thirdly, greater numbers of Fe II, Si I and Ca I lines in our study should allow better abundance determinations, and new elements (K, Sc, V, Cr and Mn) will give additional information on Galactic evolution. Lastly, the stellar age determination is based on new evolutionary tracks, and the space velocity is derived from more reliable distance and proper motion values.

In the following Sects. 2 to 6, we describe the observations and methods of analysis in details and present the derived abundances, ages and kinematics. The results are discussed in Sect. 7 and compared to those of EAGLNT. Two elements, Sc and Mn, not included in EAGLNT and represented by lines showing significant hyperfine structure (HFS) effects, are discussed in a separate paper (Nissen et al. [2000]), which includes results for halo stars from Nissen & Schuster ([1997]).