1995A&A...301..537B

Two images of the Planetary Nebula K 648 in M 15 were taken in the PC mode of the WFPC1 camera on HST in the narrow [OIII] and H_α filters, and an image in [OIII] light with the FOC. This object, never resolved before in ground-based data, is resolved in the HST images. It shows a bipolar structure: a spherical shell containing a brighter torus, of about 2.5 arcsec diameter. A surrounding fainter spherical emission extending to about 5 arcsec diameter is detected. The measured diameter allows to determine better than was previously possible the mass of the ionized gas, M_gas≃0.07±0.02M_☉. The stellar and nebular parameters derived from the HST data and archive IUE data, confirm a distance of K 648 consistent with that of M 15. The morphological structure can be studied in the resolved HST images. The observed intensity distribution is modeled with a 3-D density distribution with cylindrical symmetry, the equatorial plane being close to the line of sight. On reanalysing IUE archive data, we derive the following parameters for the central star: T_eff=35000K, consistent with most previous findings, M_*=0.58±0.02M_☉, R_*=1.9R_☉, L=4680L_☉, V_edge=-1630km/s of the wind line, V_esc=300km/s. The object is still evolving towards hotter temperatures in the constant-luminosity post-AGB phase.