Abundances in planetary nebulae near the galactic center. II - Abundance distributions

The abundances of the planetary nebulae (PNe) in the galactic bulge are presented. Comparisons are made with the disk nebulae, the solar values, and the prediction based on the observed gradients in the disk. The mean abundances in the bulge PNe cannot be predicted by the observed gradient in the disk sample. Except for helium and nitrogen, which are systematically higher in the bulge, all other elements show distributions similar to their counterparts in the disk PNe. The relatively older age of the bulge objects suggests a much faster enhancement in the bulge. The significantly higher mean N/O ratio in the bulge PNe compared to the disk PNe, is caused by the much larger fraction of low abundance nebulae with a high N/O ratio in the bulge, rather than by having -on average- more massive progenitors. This is argued since the low abundance is noticed not only in the case of oxygen, which could be largely reduced if we allow for a more intensive ON-cycle to occur, but also in the cases of neon, sulfur, and argon - the elements which are believed to be essentially untouched by the nucleosynthesis in the PN progenitors.Several lines of evidence strongly argue that in the bulge, the production rates of elements and isotopes mostly produced in low and intemediate stars relative to those of elements mainly produced in massive stars, are higher than the corresponding ratios in the solar neighbourhood or in other positions in the disk. This probably indicates an initial mass function in the bulge which is different from that in the rest of the Galaxy.An evolutionary scenario for the bulge, allowing for a more efficient star formation and a much shorter timescale to collapse than in the disk, with an initial mass function such that the fraction of massive stars is lower than in the solar neighbourhood, is then proposed to explain the exisiting data.