The Galactic distribution of planetary nebulae with different types of dust
We identify different dust features in our compilation of infrared spectra for 267 planetary nebulae (PNe) from the Spitzer, ISO, and IRAS telescopes. We classify 209 objects according to their dust type: mixed dust (MD), oxygen-rich dust (ORD), carbon-rich dust (CRD), PNe with only polycyclic aromatic hydrocarbons (PAHs) in their spectra (oPAH), and featureless (F) PNe. We study statistically the distributions of surface brightness and diameter of PNe with different types of dust as well as their distributions in the Galaxy.We find that both MD and ORD PNe are closer to the Galactic centre than CRD and oPAH PNe, and that the Galactic distributions of each pair of groups are statistically compatible, suggesting that they have similar progenitors. Since oPAH PNe have, on average, larger diameters and lower surface brightness than CRD PNe, we suggest that oPAH PNe are evolved CRD PNe. On the other hand, F PNe have the lowest surface brightness and the largest diameters, suggesting they could contain evolved PNe from any initial type of dust. Among the PNe with silicates, we find that only a few ORD PNe have just amorphous silicates in their spectra, and their distributions of Galactocentric distances and Galactic heights suggest that they had low-mass progenitors. MD PNe with both amorphous and crystalline silicates have the largest surface brightness and the smallest diameters and might be the earliest stages of PNe with the most massive and metal-rich progenitors.