Masses of the Planetary-Nebula Central Stars in the Galactic
  Globular-Cluster System from HST Imaging and Spectroscopy
  The globular cluster (GC) system of our Galaxy contains four planetary
nebulae (PNe): K 648 (or Ps 1) in M15, IRAS 18333-2357 in M22, JaFu 1 in Pal 6,
and JaFu 2 in NGC 6441. Because single-star evolution at the low stellar mass
of present-epoch GCs was considered incapable of producing visible PNe, their
origin presented a puzzle. We imaged the PN JaFu 1 with the Hubble Space
Telescope (HST) to obtain photometry of its central star (CS) and
high-resolution morphological information. We imaged IRAS 18333-2357 with
better depth and resolution, and we analyzed its archival HST spectra to
constrain its CS temperature and luminosity. All PNe in Galactic GCs now have
high-quality HST data, allowing us to improve CS mass estimates. We find
reasonably consistent masses between 0.53 and 0.58 Msun for all four objects,
though estimates vary when adopting different stellar evolutionary
calculations. The CS mass of IRAS 18333-2357, though, depends strongly on its
temperature, which remains elusive due to reddening uncertainties. For all four
objects, we consider their CS and nebular masses, their morphologies, and other
incongruities to assess the likelihood that these objects formed from binary
stars. Although generally limited by uncertainties (~0.02 Msun) in post-AGB
tracks and core mass vs. luminosity relations, the high-mass CS in K 648
indicates a binary origin. The CS of JaFu 1 exhibits compact bright [O III] and
Halpha emission, like EGB 6, suggesting a binary companion or disk. Evidence is
weaker for a binary origin of JaFu 2.
