A DETAILED ABUNDANCE ANALYSIS OF 9 HALO PLANETARY-NEBULAE

Using published spectral line data for nine halo planetary nebulae (HP Ne), we have calculated photoionization models in an attempt to gain i nsight into the physical conditions and chemical abundances of these n ebulae. The nine HPNe reported upon in this paper are K648, DdDm-1, NG C 2242, NGC 4361, PN 243.8-37.1, PN 006-41.9, M2-29, BB-1 and H4-1. Th e derived abundance ranges for the HPNe are: C 6.60-8.95, N 7.18-8.00, O 7.56-8.56, Ne 6.24-7.71, Ar 4.12-7.70, and S 4.90-7.00 [log(x) + 12 ]. The temperature range for the central stars of these nebulae is 40 000 to 140 000 K. Specifically, with a few exceptions, we find that al l nine objects exhibit subsolar O/H; most show enhanced C/O and N/O, a nd a constant Ne/O ratio. We also note the existence of comparatively larger abundance scatter in the HPNe as opposed to disc PNe, and sugge st that this is consistent with the accretion model of halo formation formulated by Searle & Zinn. In addition, we test the effects on deriv ed abundances and central star temperatures of a variety of model atmo spheres as well as blackbodies for input ionizing spectra. We find tha t nebular line strengths are relatively insensitive to atmospheric det ails; thus blackbody spectra are suitable for central star continua.