8-13 micron dust emission features in Galactic bulge planetary nebulae

A sample of 25 IR-bright planetary nebulae (PNe) towards the Galactic bulge is analysed through 8–13μm 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, 4 PNe have emission features associated with C-based grains, while the remaining 9 have O-rich dust signatures. The low fraction of C-rich PNe, <∼ 30%, contrasts with that for local PNe, around 80%, although it follows the trend for a decreasing frequency of C-rich PNe with galactocentric radius (paper I). We investigate whether the PNe discussed here are linked to the bulge stellar population (similar to type IV, or halo, PNe) or the inner Galactic disk (a young and super-metal-rich population). Although 60% of the PNe with warm dust are convincing bulge members, none of the C-rich PNe satisfy our criteria, and they are probably linked to the inner Galactic disk. 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-metal-rich (to account for the scarcity of C-rich objects). Yet the metallicities 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 here is derived from a mixed disk/bulge progenitor population and dominated by type IV PNe, as suggested by Peimbert (1992). The much higher fraction of O-rich PNe in this sample than in the solar neighbourhood should result in a proportionally greater injection of silicate grains into the inner Galactic medium.