2000AJ....120.2044A

We have obtained observations of the planetary nebula K648 in the Galactic globular cluster M15 with the Hubble Space Telescope's Wide Field Planetary Camera 2, covering an interval of 7 days. The frames provide both time-sampled broadband photometry of the central star and high-resolution images of the nebula in the light of Hα, [O III], and [N II]. In the deep narrowband images, K648 is a fairly typical double-shelled elliptical nebula, but with a bright arc at one end of the major axis that is especially prominent in [N II]; this feature is probably a collection of FLIERs (fast low-ionization emission regions). The nebula is surrounded by a faint, smooth elliptical halo, which appears undisturbed by any interaction with the interstellar medium. Adopting T_eff=40,000±3000 K based upon published spectral line analyses, and employing our new broadband optical flux data along with the known cluster distance, we find log(L/L_☉)=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_☉ 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 M_☉). The K648 central star exhibits no significant photometric variability in our data, and 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.