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BATC 15 Band Photometry of the Open Cluster NGC 188

This paper presents CCD multicolour photometry for the old open cluster NGC 188. The observations were carried out as a part of the Beijing--Arizona--Taiwan--Connecticut Multicolour 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 $t=7.5\pm 0.5$ Gyr, a distant modulus of $(m-M)_0=11.17\pm0.08$, and a reddening of $E(B-V)=0.036\pm0.010$. The radial surface density profile of NGC 188 is obtained by 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 to a power-law function $ϕ(m) \propto m^α$, the slopes of mass functions for different spatial regions are derived. We find that NGC 188 presents a slope break in the mass function. The break mass is $m_{\rm break}=0.885~M_{\odot}$. In the mass range above $m_{\rm break}$, 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 $m_{\rm break}$, 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.