The optical spectrum of the young planetary nebula Hubble 12

The remarkable young compact planetary nebula, Hubble 12, renowned for its extended envelope, which radiates the lines of molecular hydrogen excited by fluorescence, has a dense core of complex structure. We present high dispersion measurements of the spectrum from 366 to 1005 nm: if the emission lines of [N II], [O III] and [S III] originate in the same strata, the diagnostics suggest a zone with an electron density of similar to 500000 cm(-3), and T-epsilon-13600 K, but lower densities and temperatures are indicated by the auroral/nebular line ratios of other emissions such as those of [O II], [S II] and the nebular line ratios of [Cl III], [Ar IV] and especially [NI I]. The chemical composition of Hubble 12 is hard to establish, because of the great range in density and temperature of the emitting layers. We discuss arguments for and against the progenitor star as an oxygen-or metal-deficient object. A newly developed density-contrast theoretical model is applied to this bipolar object, but a convincing theoretical model must await optical monochromatic images of high spatial resolution.