Hubble Space Telescope imaging of central stars of high-excitation planetary nebulae with WFC and WFPC2

We present observations of four high excitation planetary nebulae (IC 2165, Me 2-1, NGC 2440, NGC 7027) taken with the Wide Field Camera and the Wide Field Planetary Camera 2 on board the Hubble Space Telescope. The high angu lar resolution of the instruments allows for the unambiguous identification of the central star, which was not found to be the case in most previous s tudies. In addition, the panchromatic nature of the data, which include sev eral ultraviolet bandpasses, permits us to directly determine the amount of dust extinction present along the sight line to the central star. The comb ination of these two characteristics make it possible to delineate the spec tral energy distribution of the central stars. Assuming a Planck function s pectrum and a standard interstellar extinction law with R-V = 3.1, the adop ted EB-V values are 0.40 (IC 2165), 0.15 (Me 2-1), 0.15 (NGC 2440), and 1.1 0 (NGC 7027). The wavelength coverage of our measurements is not particular ly sensitive to temperature values in the range 150,000-400,000 K. However, we do find that 200,000 K produces the best fits. Again using a Planck fun ction to represent the wavelength dependence of the stellar flux, we determ ine V magnitudes of 17.47 +/- 0.06 (IC 2165), 18.40 +/- 0.05 (Me 2-1), 17.6 3 +/- 0.16 (NGC 2440), and 16.04 +/- 0.04 (NGC 7027). Combining these resul ts with literature values for the emission-line fluxes and distances, we de rive the Zanstra temperature, crossover magnitude, radius, and luminosities for each object. The implications of these measurements for the mass and t he evolutionary state of each object is briefly discussed.