The physical structure of the planetary nebula NGC 6781

Abstract

The planetary nebula NGC 6781 was imaged in major optical emission lines. These lines allow us to construct maps of the projected, two dimensional Balmer decrement, electron density, electron temperature, ionization and abundance structure. The average electron density, determined from the [S II] lines, is ∼500 cm-3, while the electron temperature distribution, determined from the [N II] lines, is flat at ∼10 000 K. The Balmer decrement map shows that there are variations in extinction between the north and south areas of the planetary nebula. The higher extinction observed to the north of the central star is probably caused by dust spatially associated with CO emission at blue-shifted velocities. The [N II] image reveals the known optical halo, at a flux level of ∼0.2% of the strong shell emission in the east, but now the angular extent of 216″ × 190″ is much larger than previous measurements. The halo is also present in [O III], where we measure an extent of 190″ × 162″. The ionization maps indicate substantial 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 the shell in the west and the east, south-east. The typical log abundances measured for He, N, O and S are 10.97, 8.14, 8.72 and 6.90, respectively.