Hubble Space Telescope Observations of the Planetary Nebula K648 in the   Globular Cluster M15
  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. 
David R. Alves, Howard E. Bond, and Mario Livio (STScI)