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Título               : Low-ionization Structures (LISs) in Planetary Nebulae: Observations versus Models  
Conferencista: Denise Rocha Goncalves, Instituto de Astrofisica de Canarias Tenerife-Spain  
Data                  : Terça-feira, 14/08/01 15H30M  
Local                 : Auditório L.B.F. Clauzet - Água Funda  
Sumário           :
Low-ionization small-scale structures potentially contain important information about the mass loss and radiative processes that lead to the formation and development of a planetary nebula. Recently, we have obtained morphological and kinematical data for 10 PNe with the aim of studying the physical properties, origin, and evolution of the LISs therein contained. Results are both promising and puzzling since we have found LISs with notably different properties relative to each other, in terms of expansion velocities, shapes, sizes, and locations relative to the main nebular components. It appears that several physical processes have to be considered in order to account for the formation and evolution of all the different LISs that we observed.
In particular, the following basic questions are open. Are magnetic fields, either in single or binary stars, necessary for producing jets in PNe, as generally believed for jets in young stellar objects and active galactic nuclei? Which processes, even in complex systems like interacting binary stars, can produce multiple systems of highly collimated outflows expanding in directions almost perpendicular to each other as observed in some PNe? How do low-velocity collimated LISs form? Are symmetric pairs of knots recent ejecta from the central stars or fossil condensations tracing a peculiar mass-loss geometry during the asymptotic giant branch (AGB)? Are nonsymmetrical LISs formed by in situ instabilities? Taking advantage of our recent work and of the information that is spread throughout the literature, we carefully address these points analizing the 50 PNe presently known to possess this kind of structure.