Previous
abstract 
Next
abstract
We have observed three fields near the center of the post-core-collapse globular cluster NGC 6397 in $B$ and $V$, using the Faint Object Camera on the repaired Hubble Space Telescope ({\it HST}). We present accurate photometry of stars on the main sequence down to a $V$ magnitude of 24, corresponding to a stellar mass of $\sim 0.16$ ${\rm M}_{\sun}$. We present the mass function of the cluster center, and compare it with the mass function of a field five arcminutes out (Paresce, De Marchi, \& Romaniello 1994). We find a very striking degree of mass segregation:\ the main sequence near the center is dominated by stars near the turnoff mass, and few low-mass stars are present at all, compared with the outer field. This first deep observation of the center of a cusp cluster with the refurbished HST shows the remarkable extent to which energy exchanges between high- and low-mass stars have affected the stellar distribution, and demonstrates the potential of the telescope for resolving fundamental issues in cluster dynamics.
