Far-Ultraviolet Observation of the Globular Cluster NGC 6397
  We present an observational far-UV (FUV) and near-UV (NUV) study of the core
region of the globular cluster NGC 6397. The observations were obtained with
the Space Telescope Imaging Spectrograph (STIS, FUV), and the Wide Field Camera
3 (WFC3, NUV) on board the Hubble Space Telescope. Here, we focus on the UV
bright stellar populations such as blue stragglers (BSs), white dwarfs (WDs)
and cataclysmic variables (CVs). We present the first FUV-NUV color-magnitude
diagram (CMD) for this cluster. To support our classification of the stellar
populations, we compare our FUV-NUV CMD with optical data from the ACS Survey
of Galactic Globular Clusters. The FUV-NUV CMD indicates 16 sources located in
the WD area, and ten BSs within the 25"x 25" of the STIS FUV data. Eighteen
Chandra X-ray sources are located within the FUV field of view. Thirteen of
those have a NUV counterpart, of which nine sources also have a FUV
counterpart. Out of those, five sources are previously suggested CVs, and
indeed all five are located in the WD/CV region in our FUV-NUV CMD. Another CV
only has a FUV but no NUV counterpart. We also detect a NUV (but no FUV)
counterpart to the MSP located in the core of this cluster. The NUV lightcurves
of the CVs and MSP show flickering behaviour typical of CVs. We found that the
BSs and CVs are the most centrally concentrated population. This might be an
effect of mass segregation or indicate the preferred birth place of BSs and CVs
via dynamical interactions in the dense core region of GCs. HB stars are the
least centrally concentrated population and absent in the innermost area of the
core.
