Constraints on thick disc and halo parameters from HST photometry of fieldstars in the Galaxy

We analyse a sample of over 1000 stars from 32 fields imaged in the V and I bands with the Wide Field and Planetary Camera, on board of the Hubble Spa ce Telescope. The fields are located at Galactic latitudes /b/ > 15 degrees and in various directions on the sky. We consider models for the structure of the Galaxy with different choices for the main parameters governing the shape and luminosity function of the thick disk and stellar halo. Comparin g model predictions with the observed colour-magnitude diagram Ne are able to rule out an increasing or hat stellar luminosity function at the low-lum inosity end. We also rule out large values of the vertical scale height of the thick disc, z(0), finding it to be in the range 800 less than or equal to z(0) less than or equal to 1200 pc. As for the local density normalizati on, values within the range 4% less than or equal to n(0) less than or equa l to 8% seem to better reproduce the data. Our data essentially rule out a flattened stellar halo (c/a less than or similar to 0.5) or models with bot h large local normalization and effective radii.