The structure of the thick disc

The existence of the thick disc is now well established, although its spatial, chemical and kinematic properties are not clearly defined. As stressed by several authors, observations towards multiple directions are needed in order to resolve the correlations between the various model parameters. Here, we present a new survey over about 140 square degrees based on the material used for the construction of the Second Guide Star Catalog (GSC-II). A sample of about 20 000 K dwarfs up to 3 kpc selected from multiple fields towards high and intermediate latitudes has been analysed in order to trace the spatial distribution of the thick disc. Photometric parallaxes (30% accuracy) have been computed taking into account the systematics effects due to the Malmquist bias, the Eddington bias, the metallicity gradients, and the contamination of giants and binaries. A least-squares analysis of the seven fields supports a thick disc with a vertical scale-height, h_z ≃ 1.0 kpc and a relative density of about 5% and a radial scale-length h_R ≃ 3.2 kpc, and a thin disc with h_z ≃ 240 pc. Finally, the impact of the accurate astrometric measurements, including absolute parallaxes, expected from Gaia is discussed.