5.3.3. PNe in the Galactic bulge

Planetary nebulae offer one the best means to investigate the oxygen abundance in the Galactic bulge. Table 9 presents the mean oxygen abundances for PNe thought to be physically located in the Galactic bulge, using recent data. In all cases, the abundance derivations were made with T_e-based methods. The abundances derived by Ratag et al. (1997) have not been included, since many of them rely on modelling of objects with no observed constraint of T_e, and are therefore highly suspect (see discussion in Sect. 2.2.1). However, the observations from Ratag et al. (1997) were used in the compilation of line intensities by Stasinska et al. (1998), and the abundances rederived in a consistent way with T_e -based methods (for objects for which this was possible). The objects of Escudero & Costa (2001) are newly discovered PNe from the list of Beaulieu et al. (1999). We see that the abundances of PNe in the Galactic bulge have clearly higher mean values and dispersions in O/H than the extrapolation from disk PNe towards the Galactic center, which are shown in Table 10. The effect may be even stronger than suggested from these tables, since samples of PNe considered as belonging to the Galactic bulge may actually contain PNe of the disk population that are found physically in the same region as the bulge.

Planetary nebulae offer one the best means to investigate the oxygen abundance in the Galactic bulge. Table 9 presents the mean oxygen abundances for PNe thought to be physically located in the Galactic bulge, using recent data. In all cases, the abundance derivations were made with T_e-based methods. The abundances derived by Ratag et al. (1997) have not been included, since many of them rely on modelling of objects with no observed constraint of T_e, and are therefore highly suspect (see discussion in Sect. 2.2.1). However, the observations from Ratag et al. (1997) were used in the compilation of line intensities by Stasinska et al. (1998), and the abundances rederived in a consistent way with T_e -based methods (for objects for which this was possible). The objects of Escudero & Costa (2001) are newly discovered PNe from the list of Beaulieu et al. (1999). We see that the abundances of PNe in the Galactic bulge have clearly higher mean values and dispersions in O/H than the extrapolation from disk PNe towards the Galactic center, which are shown in Table 10. The effect may be even stronger than suggested from these tables, since samples of PNe considered as belonging to the Galactic bulge may actually contain PNe of the disk population that are found physically in the same region as the bulge.

Combining data on about 100 PNe in the Galactic bulge from the works of Cuisinier et al. (2000), Webster (1988), Aller & Keyes (1987) with their own data, Escudero & Costa (2001) suggest the existence of a vertical abundance gradient in the bulge, with lower O/H at high latitudes.