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 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.