The chemical evolution of the galactic disk. I: Analysis and results

With the aim to provide observational constraints on the evolution of the galactic disk, we have derived abundances of O, 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 properties of all stars have been derived from accurate kinematic data, enabling estimates to be made of the distances from the galactic center of the stars' birthplaces. Our extensive high resolution, high S/N, spectroscopic observations of carefully selected northern and southern stars provide accurate equivalent widths of up to 86 unblended absorption lines per star between 5000 and 9000 A. The abundance analysis was made with greatly improved theoretical LTE model atmospheres. Through the inclusion of a great number of iron-peak element absorption lines the model fluxes reproduce the observed UV and visual fluxes with good accuracy. A new theoretical calibration of T eff as a function of Stromgren b ― y for solar-type dwarfs has been established. The new models and T eff scale are shown to yield good agreement between photometric 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 spectroscopic analysis of weak spectral lines. Individual ages were derived from fits in the T eff ― log g plane of these somewhat evolved dwarfs 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 translated to galactic U, V and W velocity components which in turn were independently used by M. Grenon and J. Sommer-Larsen to calculate stellar orbital parameters, which agree within 5%. Following Grenon, the mean galactocentric distances were used as estimates of stellar birth places to investigate abundance gradients in the disk at different epochs. The relative iron abundances [Fe/H] and the abundance ratios relative to iron for most elements are estimated to be accurate with a standard deviation of 0.05 dex. We discuss the evolution of α elements, odd-Z elements, iron peak elements and s elements as functions of stellar 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 between age and metallicity. The strongest age-abundance correlation is found for Ba, which is interpreted as the result of the efficient s-element synthesis in low-mass AGB stars which enrich the ISM long after star formation. Metal-poor ([Fe/H] < ―0.4) stars are, as previously shown, relatively overabundant in the α elements: [α/Fe] for these metal-poor stars decreases with increasing galactocentric radius of the orbits, implying that the star formation was more vigorous and started first in the inner parts of the galactic disk. The abundances of the two odd-Z light elements sodium and aluminium increase at different rates with metallicity in the disk, which is presumably connected to their respective production in C and Ne burning in massive stars. The 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]. Possible explanations for this are discussed. Thanks to the high internal accuracy of the analysis and the large sample of programme stars, two groups of chemically mildly peculiar stars have been detected; a group of 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 peculiarities are discussed.