The physical structure of the planetary nebula NGC 6781

The planetary nebula NGC 6781 was imaged in major optical emission lines. T hese lines allow us to construct maps of the projected, two dimensional Bal mer decrement, electron density, electron temperature, ionization and abund ance structure. The average electron density, determined from the [S II] li nes, is similar to 500 cm(-3), while the electron temperature distribution, determined from the [N II] lines, is at at similar to 10 000 K. The Balmer decrement map shows that there are variations in extinction between the no rth and south areas of the planetary nebula. The higher extinction observed to the north of the central star is probably caused by dust spatially asso ciated with CO emission at blue-shifted velocities. The [N ii] image reveal s the known optical halo, at a flux level of similar to0.2% of the strong s hell emission in the east, but now the angular extent of 216" x 190" is muc h larger than previous measurements. The halo is also present in [O III], w here we measure an extent of 190" x 162". The ionization maps indicate subs tantial ionization along the caps of the ellipsoid as well as in the halo. The maps also show a sharp decrease in ionization along the outer edge of t he shell in the west and the east, south-east. The typical log abundances m easured for He, N, O and S are 10.97, 8.14, 8.72 and 6.90, respectively.