Astron. Astrophys. 321, 907-920 (1997)

7. Conclusions

The results presented in this paper demonstrate that near-infrared observations of molecular hydrogen at high spatial and spectral resolution provide valuable probes of the evolution of circumstellar envelopes in the transition from PPNe to PNe. The multiplex advantage of the instument BEAR which was used for this study allows one to observe simultaneously and under similar conditions the ionized cavity and the photo-dissociation region, the shocked layers and the high-velocity gas which characterize the inner obscured parts of transition objects.

The morphology, kinematics and excitation of molecular hydrogen have been presented for two well-known sources, the proto-panetary nebula AFGL 2688 and the young planetary nebula NGC 7027. The main results of this study are:

• The remarkable cross-like morphology of the H₂ emission seen in AFGL 2688 traces the regions of shocked gas where the high-velocity winds ram into the inner edge of the more slowly expanding AGB envelope. The H₂ line ratio and intensities, together with the large velocity dispersion observed in the four clumps are fully compatible with shock excitation in relatively dense gas (  cm^-3) and shock velocities of about 20-40 km s^-1. The excitation temperature is typically 1600 K and the estimated column densities  cm^-1.

  Along the south-north axis, the velocity dispersion is consistent with a bipolar outflow. The main axis of the outflow is aligned along the optical and near-infrared reflection nebulosity and not along an axis with a position angle of about as recently suggested by Skinner et al. (1997). The East-West H₂ velocity field most likely traces the outer edges of a dense tilted equatorial torus. The details of the geometry and the orientation of this central torus are still poorly known. From recent millimeter studies, its structure appears inhomogeneous and far from simple. High spatial resolution studies of the optically thin dust millimeter emission towards the inner regions of AFGL 2688 will help to better trace the morphology of the equatorial torus and determine its true orientation.

• The H₂ emission in NGC 7027 is distributed at the periphery of the ionized cavity and traces the thin, dense equatorial torus and the inner edges of the molecular envelope. The H₂ emission in NGC 7027 is entirely explained with excitation in a dense UV-excited PDR at the interface of the ionized and molecular gas. The H₂ velocity field shows that the kinematics of the dense, hot PDR is firmly linked with the outer envelope.

Further observations with BEAR towards other PPNe and young PNe should allow a systematic and detailed study of this rapidly evolving phase of stellar evolution. On-going studies on other sources will be presented in forthcoming papers.

© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998
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