HIGH-RESOLUTION IMAGING OF NGC-7027

We performed narrow band, optical line imaging of the planetary nebula NGC 7027 in sub-arcsecond seeing conditions. In all our filters, cent ered on the Halpha 6563, Hbeta 4861, [NII] 6583, [0III] 5007 lines and in a 6670/80 Halpha continuum band, the nebula shows a complex clumpy structure whereas the central star is well resolved in the continuum band. Image analysis allows us to obtain a very accurate map of the lo cal extinction down to A(V) = 5 and to estimate for the exciting sourc e m(V) = 16.60 +/- 0.15, leading to T congruent-to 220 000 K and L* c ongruent-to 4 700L.. Correcting our images for the extinction, the tru e annular structure of the planetary nebula is for the first time reve aled at optical wavelengths. We can definitively confirm that the opti cal maximum corresponds to a ''hole'' in the dust distribution. We tra ck the ionization status of the nebula, finding evidence for a complex scenario. The [NII] emission traces at the poles the ionization front at the edges of the HII region. A faint halo of continuum emission wi th traces of limb brightening surrounds the nebula and turns out to be coincident with the photodissociation region recently mapped by Graha m et al. (I 993). We suggest that the photodissociation region corresp onds to the progenitor of the bipolar halos observed around more evolv ed planetary nebulae. At the equatorial plane, the Halpha/[NII] and Hb eta/[OIII] ratios are enhanced in coincidence with the H-2 knots mappe d by Graham et al. (1993) whereas a ring of dust with traces of Halpha emission surrounds the HII region defining the equatorial plane of th e nebula. Inhomogeneities in the circum-nebular disk can be at the ori gin of the HII and H-2 knots, which are shock excited according to the models on the dynamical interaction of a moving medium with dense neu tral condensations. Finally, both [NII]/Halpha and [OIII]/Hbeta ratios appear enhanced in the NW quadrant of the nebula, in correspondence w ith the edges of the extinction hole. The inner HII region appears to be escaping into the circum-nebular medium widening the ''hole'' in th e extinction. In the expansion, the mechanical interaction of the stel lar wind with the neutral envelope originates the shock excitation obs erved at the edges of the cavity. NGC 7027 can be therefore considered as an extremely young planetary nebula observed in the transition pha se from pure elliptical to bipolar.