TOWARDS AN IMPROVED MODEL OF THE GALAXY Johan Holmberg

We use the Hipparcos survey to derive an improved model of the local galactic structure. The availability of parallaxes for all the stars permits direct determination of stellar distributions, eliminating the basic indeterminacy of classical methods based on star counts. The observed joint distribution of parallaxes, colours and apparent magnitudes is well reproduced by a density model of the Bahcall-Soneira type but with modified luminosity/colour distribution and scale heights. The best fit to the data is obtained with a density law perpendicular to the plane that is intermediate between the two classical forms, the exponential and the sech 2 (isothermal). The solar distance from the plane is found to be low compared to previous estimates, especially those based on optical star counts. The Hipparcos survey was intended to be a complete sample of some 50 000 stars, limited by apparent magnitude and colour. In reality, the faint part of the survey is incomplete, and the completeness ..