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We have acquired monochromatic imaging of the bipolar outflow source NGC 2440 in 12 ionic transitions using the 1.5 m Telescope at the Observatorio de San Pedro Martir, Ensenada, Baja California, Mexico. As a result, we have been able to undertake detailed analysis of the shell ionisation structure. Then, we found that excitation is high close the center, extending towards the exterior at located radial positions. Edges and ansae are zones of low excitation indicating probably shock regions, in particular the outer shell.

The conventional interpretation of the “Wilson effect”is based on the fact that, in a simple photoionization model of a thick shell in free expansion, the elements with a high ionization potential are found in the innermost part of the shell, which move slower than the outer regions where the low ionization ions are located. A real PN is however a much more complex object than in the picture above. For these reasons, we are carrying on a project aimed at studying in detail the Wilson effect in a large number of objects of different types.We present here preliminary results for two PNe, the bipolar nebula Hu 1–2 and the elliptical NGC 7662. Observation have been secured with the Utrecht Echelle Spectro-graph placed at 4.2m William Herschel Telescope, (La Palma, Canary Islands). The spatial resolution is 0.36 arcsec, and the resolving power R=46000. The results are presented in Table 1. The increase of expansion velocity with decreasing potential is clearly seen except for Hydrogen. This ion is a quite peculiar case: although its ionization potential is quite low, this element behaves also as a high excitation atom since it has only one possible ionized state. Futhermore, as we have spatial resolution we can study this relation between velocity and ionization potential in different points of the nebula.

We have obtained high spectral resolution, high signal-to-noise ratio longslit spectra of a sample of Planetary Nebulae with the purpose of studying their physical conditions, chemical composition and ionisation structure. In particular, we are interested in disentangling the effects of radiation and shock ionisation.

Sh 2-71 appears to represent a diffuse, ellipsoidal nebulosity extending over a range △α x △δ = 1.7 × 3 arcmin2. Early spectroscopy by Glushkov et al (1975), and Chopinet and Lortet-Zuckerman (1976) suggested the presence of a high excitation central star, and Kaler (1983) has more recently determined Zanstra temperatures T z (HeII) > 7.7 · 104 K. The observed central star clearly constitutes a much cooler companion, and luminosity variations in this source have been attributed to binary eclipse.

NGC 6537 is an unusual high excitation bipolar outflow source, with anomalous abundances indicative of a type I nebula. We have recently obtained a range of high resolution spectrophotometry for this source using the 2.5 m Isaac Newton Telescope (Observatorio del Roque de Los Muchachos, La Palma), together with narrow band optical imaging using the 2.6 m Nordic Optical Telescope (Observatorio del Roque de los Muchachos, La Palma). As a consequence, it is apparent that the source optical morphology is suggestive of the presence of an hyperbolic shock outflow surface, similar to those observed in other BPN (e.g. NGC 6302, Hb 5). Line ratio maps also indicate the presence of extremely strong [Nii] emission at the periphery of the outflow, whilst expansion velocities are of order ~ 400 km sec-1. These large shell expansion velocities may in turn be driven by an extremely high velocity wind, which in this case appears to extend over a range ΔV ≥ 3000 km sec-1.

NGC 6905 appears to be an unusual source in displaying conical outflow lobes superimposed upon a spheroidal inner shell.