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K 4−47: a planetary nebula excited by photons and shocks




K 4−47 is an unusual planetary nebula (PN) composed of a compact high-ionization core and a pair of low-ionization knots. Long-slit medium-resolution spectra of the knots and core are analyzed in this paper. Assuming photoionization from the central star, we have derived physical parameters for all the nebular components, and the ionization correction factors (icf) chemical abundances of the core, which appear similar to Type I PNe for He and N/O but significantly deficient in oxygen. The nebula has been further modelled using both photoionization (cloudy) and shock (mappings) codes. From the photoionization modelling of the core, we find that both the strong auroral [O iii] 4363-Å and [N ii] 5755-Å emission lines observed and the optical size of the core cannot be accounted for if a homogeneous density is adopted. We suggest that a strong density stratification, matching the high-density core detected at radio wavelengths and the much lower density of the optical core, might solve the problem. From the bow-shock modelling of the knots, on the other hand, we find that the chemistry of the knots is also represented by Type I PN abundances, and that they would move with velocities of 250–300 km s−1.