Spatial dependence of 2MASS luminosity and mass functions in the old open cluster NGC 188

Feb, 2005
13 pages
Published in:
  • Astron.Astrophys. 433 (2005) 917
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Abstract: (arXiv)
Luminosity and mass functions in the old open cluster NGC 188 are analysed by means of J and H 2MASS photometry. Within the uncertainties, the observed projected radial density profile of NGC 188 departs from the two-parameter King model in two inner regions, which reflects the non-virialized dynamical state and possibly, some degree of non-sphericity in the spatial shape of this old open cluster. Fits with two and three-parameter King models to the radial distribution of stars resulted in a core radius of 1.3 pc and a tidal radius of 21 pc. The present 2MASS analysis resulted in significant slope variations with distance in the mass function ϕ(m)m(1+χ)\phi(m)\propto m^{-(1+\chi)}, being flat in the central parts (χ=0.6±0.7\chi=0.6\pm0.7) and steep in the cluster outskirts (χ=7.2±0.6\chi=7.2\pm0.6). The overall mass function has a slope χ=1.9±0.7\chi=1.9\pm0.7, slightly steeper than a standard Salpeter mass function. Solar metallicity Padova isochrone fits to the near-infrared colour-magnitude diagram of NGC 188 resulted in an age of 7.0±1.07.0\pm1.0 Gyr. The best fit, obtained with the 7.1 Gyr isochrone, produced a distance modulus \mM=11.1\pm0.1, \ebv=0.0, and a distance to the Sun \ds=1.66\pm0.08 kpc. The observed stellar mass (in the range 0.98 \ms - 1.08 \ms) in NGC 188 is \mobs=380\pm12 \ms. A simple extrapolation of the observed overall mass function to stars with 0.08 \ms resulted in a total present mass of \mtot\sim(1.8\pm0.7)\times10^4 \ms. On the other hand, for a more realistic initial mass function which flattens in the low-mass range, the total mass in NGC 188 drops to \mtot\sim(3.8\pm1.6)\times10^3 \ms.
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