A detailed abundance analysis of nine halo planetary nebulae

Using published spectral line data for nine halo planetary nebulae (HPNe), we have calculated photoionization models in an attempt to gain insight into the physical conditions and chemical abundances of these nebulae. The nine HPNe reported upon in this paper are K648, DdDm-1, NGC 2242, NGC 4361, PN 243.8-37.1, PN 006-41.9, M2-29, BB-1 and H4-1. The 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 all nine objects exhibit subsolar O/H; most show enhanced C/O and N/O, and a constant Ne/O ratio. We also note the existence of comparatively larger abundance scatter in the HPNe as opposed to disc PNe, and suggest that this is consistent with the accretion model of halo formation formulated by Searle & Zinn. In addition, we test the effects on derived abundances and central star temperatures of a variety of model atmospheres as well as blackbodies for input ionizing spectra. We find that nebular line strengths are relatively insensitive to atmospheric details; thus blackbody spectra are suitable for central star continua.