The Initial Mass Function and the Surface Density Profile of NGC 6231
  We have performed new wide-field photometry of the young open cluster NGC
6231 to study the shape of the initial mass function (IMF) and mass
segregation. We also investigated the reddening law toward NGC 6231 from
optical to mid-infrared color excess ratios, and found that the
total-to-selective extinction ratio is Rv = 3.2, which is very close to the
normal value. But many early-type stars in the cluster center show large color
excess ratios. We derived the surface density profiles of four member groups,
and found that they reach the surface density of field stars at about 10',
regardless of stellar mass.
  The IMF of NGC 6231 is derived for the mass range 0.8 -- 45 Msun. The slope
of the IMF of NGC 6231 (Gamma = -1.1 +/- 0.1) is slightly shallower than the
canonical value, but the difference is marginal. In addition, the mass function
varies systematically, and is a strong function of radius - it is is very
shallow at the center, and very steep at the outer ring suggesting the cluster
is mass segregated. We confirm the mass segregation for the massive stars (m >~
8 Msun) by a minimum spanning tree analysis. Using a Monte Carlo method, we
estimate the total mass of NGC 6231 to be about 2.6 (+/- 0.6) x 10^3 Msun. We
constrain the age of NGC 6231 by comparison with evolutionary isochrones. The
age of the low-mass stars ranges from 1 to 7 Myr with a slight peak at 3 Myr.
However the age of the high mass stars depends on the adopted models and is 3.5
+/- 0.5 Myr from the non- or moderately-rotating models of Brott et al. as well
as the non-rotating models of Ekstr\"om et al. But the age is 4.0 -- 7.0 Myr if
the rotating models of Ekstr\"om et al. are adopted. This latter age is in
excellent agreement with the time scale of ejection of the high mass runaway
star HD 153919 from NGC 6231, albeit the younger age cannot be entirely
excluded.
