Galactic structure studies with BATC star counts

¹ National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, PR China e-mail: dch@vega.bac.pku.edu.cn
² Department of Physics, National Cheng Kung University, Taiwan 70148, Taiwan

Corresponding author: Cuihua Du, dch@vega.bac.pku.edu.cn

Received: 16 January 2003
Accepted: 21 March 2003

Abstract

We report the first results of star counts carried out with the National Astronomical Observatories (NAOC) 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 (), which lie in the field with central coordinates and (J2000) (Galactic coordinates: ). The field of view is 0.95 deg², and the spatial scale was . 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 pc and of the thick disk to be pc, respectively, with a local density of of the thin disk. The errors of scale heights and the corresponding space number density normalization are estimated at a confidential level. The density law for the Galactic halo population is also investigated. We find that the observed counts support an axial ratio of for a de Vaucouleurs 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.