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 pr esent 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 th e same strata, the diagnostics suggest a zone with an electron density of similar to 500000 cm(-3), and T-epsilon-13600 K, but lower densiti es and temperatures are indicated by the auroral/nebular line ratios o f 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 compo sition of Hubble 12 is hard to establish, because of the great range i n density and temperature of the emitting layers. We discuss arguments for and against the progenitor star as an oxygen-or metal-deficient o bject. 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.