THE CHEMICAL EVOLUTION OF THE GALACTIC DISK .1. ANALYSIS AND RESULTS

With the aim to provide observational constraints on the evolution of the galactic disk, we have derived abundances of 0, Na, Mg, Al, Si, Ca , Ti, Fe, Ni, Y, Zr, Ba and Nd, as well as individual photometric ages , for 189 nearby field F and G disk dwarfs. The galactic orbital prope rties of all stars have been derived from accurate kinematic data, ena bling estimates to be made of the distances from the galactic center o f the stars' birthplaces. Our extensive high resolution, high S/N, spe ctroscopic observations of carefully selected northern and southern st ars provide accurate equivalent widths of up to 86 unblended absorptio n lines per star between 5000 and 9000 angstrom. The abundance analysi s was made with greatly improved theoretical LTE model atmospheres. Th rough the inclusion of a great number of iron-peak element absorption lines the model fluxes reproduce the observed UV and visual fluxes wit h good accuracy. A new theoretical calibration of T(eff) as a function of Stromgren b - y for solar-type dwarfs has been established. The ne w models and T(eff) scale are shown to yield good agreement between ph otometric and spectroscopic measurements of effective temperatures and surface gravities, but the photometrically derived very high overall metallicities for the most metal rich stars are not supported by the s pectroscopic analysis of weak spectral lines. Individual ages were der ived from fits in the T(eff) - log g plane of these somewhat evolved d warfs to isochrones by VandenBerg (1985). We expect the uncertainties in the relative ages to be about 25%, although the absolute errors may be larger. Distances, proper motions and radial velocities were trans lated to galactic U, V and W velocity components which in turn were in dependently used by M. Grenon and J. Sommer-Larsen to calculate stella r orbital parameters, which agree within 5%. Following Grenon, the mea n galactocentric distances were used as estimates of stellar birth pla ces to investigate abundance gradients in the disk at different epochs . The relative iron abundances [Fe/H] and the abundance ratios relativ e to iron for most elements are estimated to be accurate with a standa rd deviation of 0.05 dex. We discuss the evolution of alpha elements, odd-Z elements, iron peak elements and s elements as functions of stel lar age and orbital properties in terms of nucleosynthesis in massive stars, in supernovae of Types Ia and II, and in AGB stars. There is a considerable variation in the metallicities of stars formed at a given time in the disk, which means that there is only a weak correlation b etween age and metallicity. The strongest age-abundance correlation is found for Ba, which is interpreted as the result of the efficient s-e lement synthesis in low-mass AGB stars which enrich the ISM long after star formation. Metal-poor ([Fe/H] < -0.4) stars are, as previously s hown, relatively overabundant in the alpha elements: [alpha/Fe] for th ese metal-poor stars decreases with increasing galactocentric radius o f the orbits, implying that the star formation was more vigorous and s tarted first in the inner parts of the galactic disk. The abundances o f the two odd-Z light elements sodium and aluminium increase at differ ent rates with metallicity in the disk, which is presumably connected to their respective production in C and Ne burning in massive stars.Th e scatter, at a given age and mean distance from the galactic center, in relative abundances like [Si/Fe] is only about 0.05 or less, which is about 4 times less than the corresponding scatter in [Fe/H]. Possib le explanations for this are discussed. Thanks to the high internal ac curacy of the analysis and the large sample of programme stars, two gr oups of chemically mildly peculiar stars have been detected; a group o f metal rich stars appear to be enriched in Na, Mg and Al relative to other elements, another group of dwarfs are enriched in s-elements as previously reported by Tomkin et al. (1989). The origins of these pecu liarities are discussed.