2015AJ....150...61W

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 Å. 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). 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 C₀ = log(R_t/R_c) = 1.07. Fitting the mass function (MF) to a power-law function φ(m) ∝ m^α, 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_☉. In the mass range above m_break, the slope of the overall region is α = -0.76. The slope of the core region is α = 1.09, and the slopes of the external regions are α = -0.86 and α = -2.15, respectively. In the mass range below m_break, these slopes are α = 0.12, α = 4.91, α = 1.33, and α = -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.