c © ESO 2003 Astronomy &Astrophysics Galactic structure studies with BATC star counts

Abstract. We report the first results of star counts carried out with the National Astronomical Observatories (NAOC) 60/90 cm Schmidt Telescope in 15 intermediate-band filters from 3000 to 10 000 Å in the BATC survey. We analyze a sample of over 1400 main sequence stars (14 V 21), which lie in the field with central coordinates RA = 09h53m13s.30 and Dec = 474900000.0 (J2000) (Galactic coordinates: l = 169.95 , b = 49.80). The field of view is 0.95 deg2, and the spatial scale was 100.67. Since star counts at high galactic latitudes are not strongly related to the radial distribution, they are well suited to study the vertical distribution of the Galaxy. In our model, the distribution of stars perpendicular to the plane of the Galaxy is given by two exponential disks (thin disk plus thick disk) and a de Vaucouleurs halo. Also, based on star counts, we derive the scale heights of the thin disk to be 320+14−15 pc and of the thick disk to be 640+30−32 pc, respectively, with a local density of 7.0 1% of the thin disk. The errors of scale heights and the corresponding space number density normalization are estimated at a 68% confidential level. The density law for the Galactic halo population is also investigated. We find that the observed counts support an axial ratio of c/a 0.6 for a de Vaucouleurs r1/4 law, implying a more flattened halo. We consider that it is possible that the halo has two subpopulations–a flattened inner halo and a spherical outer halo in the Milky Way, and such a halo model might resolve many of the divergences in star count results. We also derive the stellar luminosity function (SLF) for the thin disk, and it partly agrees with the Hipparcos luminosity function. Key words. Galaxy: structure – stars: luminosity function, mass function 1