An observational determination of the evolution of planetary nebulae

For a sample of 23 Galactic planetary nebulae, selected on the basis of distance certainty and temperature of the central stars being below T < 70; 000 K, central star Zanstra luminosities are determined. This has been done by obtaining magnitudes, interstellar extinctions and Zanstra temperatures. The magnitudes are determined from CCD images taken through a narrow band Hr -lter centered at a wavelength 6648A and transposing the uxes found towards the visual. The extinctions are found by at the same time obtaining images taken through a H -lter centered at 6566A. The ratio of the resulting nebular line H -uxes and 6cm or 21cm radio ux densities gives the extinction. The extinctions can be deduced from catalogued H=H -ratios and the Galactic dust map of Schlegel et al. (1998) as well. The Zanstra temperatures are obtained from the ratio of the ionizing and the visual photons emitted by the star, F (H )(6566)=Fvis(5450). For this method to work it is assumed that the central star radiates as a blackbody and the optical depth of the hydrogen ionizing radiation is greater than unity. The positions of the central stars along with their ages are plotted in a Hertzsprung-Russelldiagram for each of the four methods used to determine the extinction. The ages are determined by dividing the radii of the planetary nebulae with their expansion velocities. Then this observational determination of the evolution of planetary nebulae is compared to the the