Mass segregation in young large Magellanic Cloud clusters. I. NGC 2157

We have carried out Wide Field Planetary Camera 2 V-and I-band imaging of the young LMC cluster NGC 2157. Construction of a color-magnitude diagram and isochrone fitting yield an age of tau = 10(8) yr, a reddening E(B-V) = 0.1, and a distance modulus of 18.4 mag. Our data cover the mass range 0.75 M. less than or equal to m less than or equal to 5.1 M.. We find that the cluster mass function changes significantly from the inner regions to the outer regions, becoming steeper (larger number of low-mass stars relative to high-mass stars) at larger radii. The age of NGC 2157 is comparable to its two-body relaxation timescale only in the cluster core. The observed steepening of the mass function at larger radii is therefore most likely an initial condition of the cluster stars, Such initial conditions are predicted in models of cluster star formation in which dissipative processes act more strongly upon more massive stars.