BATC 15 BAND PHOTOMETRY OF THE OPEN CLUSTER NGC 188

This paper presents CCD multicolor photometry for the old open cluster NGC 188. The observations were carried out as part of the Beijing-Arizona-Taiwan-Connecticut Multicolor Sky Survey from 1995 February to 2008 March, using 15 intermediate-band filters covering 3000-10000 angstrom. By fitting the Padova theoretical isochrones to our data, the fundamental parameters of this cluster are derived: an age of t = 7.5 +/- 0.5 Gyr, a distance modulus of (m-M)(0) = 11.17 +/- 0.08, and a reddening of E (B-V) = 0.036 +/- 0.010. The radial surface density profile of NGC 188 is obtained using the star count. By fitting the King model, the structural parameters of NGC 188 are derived: a core radius of R-c = 3'.80, a tidal radius of R-t = 44'.78, and a concentration parameter of C-0 = log(R-t/R-c)= 1.07. Fitting the mass function (MF) to a power-law function phi(m) proportional to m(alpha), the slopes of the MFs for different spatial regions are derived. We find that NGC 188 presents a slope break in the MF. The break mass is m(break) = 0.885 M-circle dot. In the mass range above m(break), the slope of the overall region is alpha = -0.76. The slope of the core region is alpha = 1.09, and the slopes of the external regions are alpha = -0.86 and alpha = -2.15, respectively. In the mass range below m(break), these slopes are alpha = 0.12, alpha = 4.91, alpha = 1.33, and alpha = -1.09, respectively. The mass segregation in NGC 188 is reflected in the obvious variation of the slopes in different spatial regions of this cluster.