4 Summary

We have reported the first detection of secondary structures (faint halo, outermost emissions) around some galactic planetary nebulae using narrow bandpass interference filters centred at H$\alpha$, [NII]6583 and [OIII]5007 Å. Alike NGC 6720 (Ring Nebula), such outer structures, which give the PNe a mulptiple-shell morphology (Chu et al. 1987), could be understood as relics of primary ejections when the PN progenitors experience the AGB-phase. Alternatively, these newly detected emissions could result from the projection of the loop/bubble structures situated in each side of the nebula "waist'' (which generally appear as elliptical/rectangular bright cores, e.g. Sh 1-89 in Hua 1997). This interpretation is supported by the ellipsoidal shell models proposed by Zhang & Kwok (1998) taking into account the pole-to-equator density gradient. Viewed along its major axis, Sh 1-89 would display a multiple-shell structure, of which the Ring nebula (NGC 6720) is a good representative. The slight asymmetrical shape (interpreted as due to the influence of the interstellar medium, Soker 1997) would result from the angle between line-of-sight and the PN major axis. Seen edge-on, NGC 6720 would display bipolarity with a dumbbell-shape, alike Sh 1-89 (Hua 1997), or a double-cone-shape, alike SaWe 3 in the present study. Meanwhile, such outer emissions raise the problem of overall ionized mass for planetary nebulae which depends upon several parameters so far not well known. In the majority of cases, $N\rm _e$ is not measured, and the distance is very poorly determined. Moreover, most of the PNe are far from homogeneous. For this reason, we have provided ionized masses in terms of $\alpha$ $\times$ $\epsilon$ $N\rm _e$, $\alpha$ being a numerical factor which mainly depends upon the PN distance, and H$\alpha$ radius (numerical values in Table 2, Col. 11). Absolute calibrations are available for the PNe reported in the present study in FITS files for further investigations. In addition, a new emission nebula was detected in the field of NGC 3699, which presumably emits stronger in [NII] and [OIII] than in H$\alpha$.

Acknowledgements

CTH would like to express his thanks to Drs. J.B. Hearnshaw, W. Tobin (University of Canterbury, Christchurch) and M. Clark (Lake Tekapo Superintendant) for their kind hospitality during several observing runs at MJUO, and INSU-CNRS for travel grants. We are indebted to the MSSO Program Committee for allocation of the ATT 2.3 metre telescope at Siding Spring Observatory. We thank Prof. S. Kwok of the University of Calgary for his pertinent comments on using the radio data to compare with the optical measurements. We thanks Prof. J. Lequeux for his advices to improve the manuscript.