Chemical Abundances in Wolf-Rayet Ring Nebulae: Implications for Nucleosynthesis and Stellar Evolution

Resumen

The results of detailed, high-spatial-resolution spectroscopy for a sample of eleven nebulae surrounding Wolf-Rayet stars: S308, NGC 2359, NGC 3199, Anon (MR 26), RCW 78, RCW 104, G 2.4+1.4, M 1-67, L 69.8+1.74, Anon (MR 100), and NGC 6888, are presented. The study of the ionization structure, using diagnostic diagrams, indicates that photoionization is the main source of excitation in the observed sample. Physical conditions and chemical abundances have been derived for as many of the observed slit positions as possible in each nebula. The observed differential abundances, relative to H II regions at the same galactocentric distances, allow us to infer whether any chemical enrichment has occurred in the nebulae. Analyzing this parameter has allowed a chemical classification of the whole sample, distinguishing three different abundance patterns: stellar ejecta, diluted nebula, and H II region, which are correlated with other physical properties of the nebulae and central stars. The most enriched objects are overabundant in N and He, and O deficient--a behavior that can only be explained if they consist of almost pure stellar ejecta and, hence, reflect the products of CNO nucleosynthesis. A new observational method has been designed to facilitate a chemo-dynamical study of objects which have emission lines. It has been applied successfully to NGC 6888, permitting us to isolate chemically enriched nebular matter in expansion, coming from the central star, with respect to the surrounding interstellar medium, which shows abundances typical of the solar vicinity. Detailed photoionization modelling has been performed for eight of the sample nebulae. The objects have been modelled ''physically'' using photoionization codes taking into account their physical and chemical properties and the most recent Wolf-Rayet model atmospheres for the ionizing flux distribution. This modelling provided the fundamental parameters for the central stars: luminosity, effective temperature, and radius, as well as properties for the associated nebulae: number of ionizing photons, ionization parameter, mass, and optical depths. A correlation between effective temperature and spectral subtype of the central star has been found. The effect of line blanketing on the results has been explored, finding that it can be very important for the coolest stars. For the first time, the radius of the central star and its spectral subtype appears to be well correlated. Comparing the results with evolutionary models for massive stars it is found that (a) the position of the central stars of the modelled nebulae in the H-R diagram indicates that their initial masses are, in general, in the range 25-40 solar mass; (b) the chemical abundances found in the most enriched objects are similar to the expected nucleosynthesis predicted for the surface of stars with initial masses in the range 25-40 solar mass, at a particular point towards the Wolf-Rayet stage. The results suggest that the bulk of the enriched material should be ejected at, or very near to, the end of the cool hypergiant or red-supergiant phase. The empirical stellar yields in He and N for the most enriched objects in the sample has been estimated and compared with those for other reference objects such as Of shells and planetary nebulae. A substantial difference between the enrichment produced by massive stars in stellar-ejecta ring nebulae and that by less massive stars in planetary nebulae has been found, showing a relative overproduction of helium in massive stars.