8-13 mu m dust emission features in Galactic bulge planetary nebulae

A sample of 25 infrared-bright planetary nebulae (PNe) towards the Galactic bulge is analysed through 8-13 mum spectroscopy. The classification of the warm dust emission features provides a measure of the C/O chemical balance , and represents the first C/O estimates for bulge PNe. Out of 13 PNe with identified dust types, four PNe have emission features associated with C-ba sed grains, while the remaining 9 have O-rich dust signatures. The low frac tion of C-rich PNe, less than or similar to 30 per cent, contrasts with tha t for local PNe, around similar to 80 per cent, although it follows the tre nd for a decreasing frequency of C-rich PNe with galactocentric radius (Pap er I). We investigate whether the PNe discussed here are linked to the bulg e stellar population (similar to type IV, or halo, PNe) or the inner Galact ic disc (a young and super-metal-rich population). Although 60 per cent of the PNe with warm dust are convincing bulge members, none of the C-rich PNe satisfies our criteria, and they are probably linked to the inner Galactic disc. In the framework of single star evolution, the available information on bulge PNe points towards a progenitor population similar in age to that of local PNe (type I PNe are found in similar proportions), but super-meta l-rich (to account for the scarcity of C-rich objects). Yet the metalliciti es of bulge PNe, as inferred from [O/H], fail to reach the required values - except for the C-rich objects. It is likely that the sample discussed her e is derived from a mixed disc/bulge progenitor population and dominated by type IV PNe, as suggested by Peimbert. The much higher fraction of O-rich PNe in this sample than in the solar neighbourhood should result in a propo rtionally greater injection of silicate grains into the inner Galactic medi um.