  We have obtained observations of the planetary nebula K648 in the Galactic
globular cluster M15 with the Hubble Space Telescope's WFPC2 camera, covering
an interval of 7 days. The frames provide both time-sampled broad-band
photometry of the central star and high-resolution images of the nebula in the
light of H-alpha, [OIII], and [NII]. In the deep narrow-band images, K648 is a
fairly typical double-shelled elliptical, but with a bright arc at one end of
the major axis that is especially prominent in [NII]; this feature is probably
a collection of FLIERs. The nebula is surrounded by a faint, smooth elliptical
halo, which appears undisturbed by any interaction with the interstellar
medium. Adopting Teff = 40,000 +- 3,000 K based upon published spectral-line
analyses, and employing our new broad-band optical flux data along with the
known cluster distance, we find log L/L_sun = 3.78 +- 0.08 for the K648 central
star. Theoretical post-asymptotic-giant-branch evolutionary tracks imply a mass
of 0.60 +- 0.02 M_sun for this luminosity, which is significantly higher than
the mean mass of white dwarfs in globular clusters and the halo field (0.50 +-
0.02). The K648 central star exhibits no significant photometric variability in
our data; thus we find no direct evidence of a close binary companion. We
suggest that the progenitor of K648 experienced mass augmentation in a
close-binary merger, allowing it to evolve to a remnant of higher mass than
those of the single stars in the cluster.
