CCD IMAGERY OF PLANETARY NUCLEI - THE PROPERTIES OF THE CENTRAL STARS OF NGC-2440 AND NGC-7027

We report on the first results of a program of narrow-band imagery of planetary nebulae suspected of harboring exceedingly hot (T > 300,000 K), "massive" (M > 1.0 M⊙) central stars. Two of the most likely candidates are the nuclei of NGC 2440 and NGC 7027, for which Zanstra temperatures ∼350,000 K have been reported, based on narrow-band continuum imaging data. However, even with carefully chosen passbands, the continuum nebular emission for such objects swamps the flux from the central star. We have therefore combined narrow-band CCD imaging (in Hβ, in He II 4686 Å, and in the line-free continuum) with image-subtraction techniques in order to isolate the central stars from the background nebulae, and to derive their magnitudes and Zanstra temperatures. We measure V = 17.6 ± 0.15 for the nucleus of NGC 2440 and V = 16.18 ± 0.2 for the nucleus of NGC 7027, leading to a value of Teff ≈ 200,000 K for the central stars of both nebulae. Using published evolutionary tracks for planetary nuclei, we estimate central star masses of 0.62-0.70 M⊙ for NGC 2440 and 0.65-0.70 for NGC 7027. These values are not much higher than the norm for planetary nuclei, which is estimated at 0.60 M⊙.