INAF - Osservatorio Astronomico di Padova -

 Expanding Nebulae

Figure 1:The monochromatic images of the Ring Nebula in He II(upper-left), [O III] (upper-right), [N II] (lower-left) and multi-color (lower-right) as derived by our 3-D recovery program applied to high-resolution spectra. North is up and East to the left.

  The presence of circumstellar and interstellar expanding gas is a typical signature of the instability phases in stellar evolution, characterized by large, prolonged mass-loss (planetary nebulae, symbiotic star nebulae, shells around Wolf-Rayet and LBV (luminous blue variable) stars), or even an explosive event (nova and supernova remnants). The mass-loss of evolved stars occupies a strategic ground between stellar and interstellar physics: (i) it raises fundamental astrophysical problems (e. g. origin and structure of winds, formation and evolution of dust, synthesis of complex molecules), (ii) it plays a decisive role in the final stages of stellar evolution, and (iii) it is crucial for galactic enrichment in light and heavy elements. The tomographic 3-D analysis developed at the Astronomical Observatory of Padova overcome the stumbling block of de-projection, rebuild the true spatial structure of each nebula, map at unprecedented accuracy the expansion velocity field and the main physical parameters (electron density, electron temperature, turbulence, ionic and chemical abundances), and allow us a direct comparison with the current theoretical evolutionary models, the detailed hydro-dynamical simulations, and the updated photo-ionization codes

Figure 2:The optical appearance of NGC 6720 (blue=He II, green=[O III],red=[N II]) for a rotation through the North-South axis centered on the exciting star. The leftmost panel, (00,00), corresponds to the re-built nebula as seen from the Earth (North is up and East to the left).

 Let's take, for example, NGC 6720, the famous Ring Nebula in Lyra (Figs. 1 and 2). The tomographic and 3-D analysis applied to high-resolution spectra shows that the main shell of the Ring Nebula is a tri-axial ellipsoid (radii 0.10, 0.13 and 0.20 pc) seen nearly pole-on and expanding in a ballistic mode. The central star characteristics, combined with the nebular age, indicate that the post-AGB star is approaching the white dwarf cooling sequence. The equator of the Ring Nebula is optically thick and much denser than the optically thin poles. The inner halo surrounding NGC 6720 represents the pole-on projection of the AGB wind at high latitudes (i. e. circumpolar) directly ionized by the central star, whereas the outer, fainter and circular halo is the projection of the recombining AGB wind at mean-to-low latitudes, shadowed by the main nebula. At present, we are: Concluding the high-resolution spectroscopy survey of galactic planetary nebulae in both hemispheres (carried out with ESO NTT+EMMI and TNG+SARG), Extending the high-resolution spectroscopy survey to other classes of expanding nebulae (i. e. symbiotic star nebulae, bubbles around LBV and Wolf-Rayet stars, nova shells and supernova remnants), Combining HST imagery (two HST orbits have been assigned to our proposal for Cycle 16) with tomography and 3-D recovery for a representative sample of planetary nebulae in different evolutionary phases. This will provide the overall and small-scale ionic, chemical and spatio-kinematic properties of the ionized gas at unprecedented accuracy (FLIERs, SLOWERs, jets, BRETs, knots, shocked regions etc.).

People: F. Sabbadin, S. Benetti, E. Cappellaro, R. Ragazzoni, M. Turatto

Collaboration: G. Ferland (Kentucky Univ.,USA), W. J. Henney (UNAM Campus Morelia, Mexico), C. R. O'Dell (Vanderbilt Univ., USA), D. Schonberner (Astrophysikalisches Institut Potsdam, Germany), G. Umana and C. Trigilio (Istituto di Radioastronomia, Stazione VLBI di Noto)

Publications: