Hubble Space Telescope Imaging of Central Stars of High-Excitation Planetary Nebulae with WFC and WFPC2

Abstract

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 angular resolution of the instruments allows for the unambiguous identification of the central star, which was not found to be the case in most previous studies. In addition, the panchromatic nature of the data, which include several ultraviolet bandpasses, permits us to directly determine the amount of dust extinction present along the sight line to the central star. The combination of these two characteristics make it possible to delineate the spectral energy distribution of the central stars. Assuming a Planck function spectrum and a standard interstellar extinction law with R_V=3.1, the adopted E_B-V values are 0.40 (IC 2165), 0.15 (Me 2-1), 0.15 (NGC 2440), and 1.10 (NGC 7027). The wavelength coverage of our measurements is not particularly 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 function to represent the wavelength dependence of the stellar flux, we determine V magnitudes of 17.47+/-0.06 (IC 2165), 18.40+/-0.05 (Me 2-1), 17.63+/-0.16 (NGC 2440), and 16.04+/-0.04 (NGC 7027). Combining these results with literature values for the emission-line fluxes and distances, we derive the Zanstra temperature, crossover magnitude, radius, and luminosities for each object. The implications of these measurements for the mass and the evolutionary state of each object is briefly discussed.