Models of the planetary nebulae IC 2003, NGC 3242, 6210 and 7009 : constraints on the ionizing radiation of the central star.

Abstract

Using ultraviolet, optical, and infrared emission line data, ionization models for four planetary nebulae are constructed. In the fitting procedure, we place emphasis on [O III] λ500.9 nm and [O III] λ436.3 nm to reproduce both the ionization and thermal balance in the O⁺⁺ zone. The ratios C IV λ155.0 nm/C III] λ190.8 nm, N IV] λ148.7 nm/N III] λ175.0 nm, and [Ne IV] λ242.4 nm/[Ne III] λ386.8 nm are sensitive to the geometry of the nebula, and can be fitted simultaneously by taking into account a large central cavity. The nebular emission lines of NGC 6210 can be reproduced by assuming a blackbody spectrum for the central star. In NGC 7009, 3242, and II 2003, blackbody stellar flux distributions yield too high electron temperatures. This can be explained by deviations from a blackbody flux - one needs a stellar temperature lower than the He II Zanstra temperature plus an excess of He⁺ ionizing radiation. One may also introduce density fluctuations in the nebula. We discuss the ambiguity of these two explanations.