Hubble Space Telescope and ground-based spectroscopy of K648 in M15

We derive some element abundances for the central star of the planetary nebula K648 from analysis of Hubble Space Telescope Faint Object Spectrograph spectra (1200-3300 Angstrom). Carbon is found to be approximately 4 times overabundant with respect to solar values, while oxygen is underabundant and silicon about solar, and He/H = 0.6. The stellar continuum indicates a reddening of E(B-V) = 0.08 and is well approximated by a synthetic model with T-eff = 37000K and log g = 4.0, as found previously by Heber, Dreizler, & Werner. The scaling of the synthetic model with the measured UV flux indicates R= 1.3 R-circle dot, and thus log (L/L-circle dot) = 3.45, M = 0.62 M-circle dot, assuming a distance of D=10.3 kpc. The C IV exhibits a wind profile, from which a terminal wind velocity of V-infinity = 1600 +/- 150 km s(-1) is derived and a column density of N-i(C IV) = 6 +/- 3x10(15) cm(-2). From ground-based spectra obtained at the William Hershel 4.2 m telescope we derive the first measurement of the expansion velocity for the nebula: V-exp ([N II]) =11.9 km s(-1) and V-exp(H alpha) = 16.7 km s(-1). Combined with the nebular radius measured from HST imaging in our previous paper, the velocity yields an expansion age of the object Age(kinematic) greater than or equal to 4270 yr. This value is slightly higher than predictions from post-AGB evolutionary models for a approximate to0.6 M-circle dot He-burning remnant. The photospheric overabundance of He and C, as well as the low luminosity of the central star, are consistent with a post-AGB object that experienced a late thermal pulse and consequent third dredge-up, and which is now in a luminosity-dip He-burning phase. The progenitor mass is fairly low.