The chemical compositions of Galactic disc F and G dwarfs

Photospheric abundances are presented for 27 elements from carbon to europium in 181 F and G dwarfs from a differential local thermodynamic equilibrium (LTE) analysis of high-resolution and high signal-to-noise ratio spectra. Stellar effective temperatures (T-eff) were adopted from an infrared flux method calibration of Stromgren photometry. Stellar surface gravities (g) were calculated from Hipparcos parallaxes and stellar evolutionary tracks. Adopted T-eff and g values are in good agreement with spectroscopic estimates. Stellar ages were determined from evolutionary tracks. Stellar space motions (U, V, W) and a Galactic potential were used to estimate Galactic orbital parameters. These show that the vast majority of the stars belong to the Galactic thin disc. Relative abundances expressed as [X/Fe] generally confirm previously published results. We give results for C, N, O, Na, Mg, Al, Si, S, K, Ca, Sc, Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Sr, Y, Zr, Ba, Ce, Nd and Eu. The alpha elements - O, Mg, Si, Ca and Ti - show [alpha/Fe] to increase slightly with decreasing [Fe/H]. Heavy elements with dominant contributions at solar metallicity from the s-process show [s/Fe] to decrease slightly with decreasing [Fe/H]. Scatter in [X/Fe] at a fixed [Fe/H] is entirely attributable to the small measurement errors, after excluding the few thick disc stars and the s-process-enriched CH subgiants. Tight limits are set on 'cosmic' scatter. If a weak trend with [Fe/H] is taken into account, the composition of a thin disc star expressed as [X/Fe] is independent of the star's age and birthplace for elements contributed in different proportions by massive stars (Type II supernovae), exploding white dwarfs (Type Ia supernovae) and asymptotic red giant branch stars. By combining our sample with various published studies, comparisons between thin and thick disc stars are made. In this composite sample, thick disc stars are primarily identified by their V-LSR in the range -40 to -100 km s(-1). These are very old stars with origins in the inner Galaxy and metallicities [Fe/H] less than or equal to -0.4. At the same [Fe/H], the sampled thin disc stars have V-LSR similar to0 km s(-1), and are generally younger with a birthplace at about the Sun's Galactocentric distance. In the range -0.35 greater than or equal to [Fe/H] greater than or equal to -0.70, well represented by present thin and thick disc samples, [X/Fe] of the thick disc stars is greater than that of thin disc stars for Mg, Al, Si, Ca, Ti and Eu. [X/Fe] is very similar for the thin and thick disc for - notably - Na and iron-group elements. Barium ([Ba/Fe]) may be underabundant in thick relative to thin disc stars. These results extend previous ideas about composition differences between the thin and thick disc.