A MODEL FOR THE FORMATION, EVOLUTION AND STRUCTURE OF THE SOLAR CYLINDER

Models of the formation, evolution and structure of the solar cylinder are constructed on the basis of a large set of observational data on mainly late-type stars. Star formation in the disc is modelled with a simple, phenomenological prescription of the Schmidt type: SIGMA is-p roportional-to SIGMA(g)N, N = 1-2. Models with a threshold for star fo rmation are also considered. Infall of halo gas is assumed to happen o n a characteristic time-scale t(i). The results obtained are largely i ndependent of the star formation prescription. For the best-fitting mo del (N = 1), we find that the age of the solar cylinder is t0 = 11.8 /- 1.0 Gyr and that t(i) = 3.4 +/- 0.4 Gyr. We find that the upper lim it to the local disc dark matter fraction P is 0.02 +/- 0.07 and that the old disc and the thick disc are discrete components, with the tran sition between old disc and thick disc occurring about 8 Gyr ago at a metallicity of [Fe/H] = -0.6. We furthermore find that the star format ion rate has been roughly constant over the lifetime of the solar cyli nder and that the massive halo of our Galaxy is quite round (q(H) > 0. 5). It is suggested that the thick disc formed after a merger between a satellite galaxy with a mass of a few times 10(9) M. and the Galacti c disc. In conjunction with the Galaxy formation theory of Sommer-Lars en (SL), the present work indicates that the age of the metal-weak hal o is 14 +/- 1 Gyr. This is in very good agreement with recent estimate s of the absolute ages of metal-weak Galactic globular clusters. The i nfall time-scale derived in the present work is in reasonable agreemen t with the one predicted by the SL Galaxy formation theory.