The Galactic disc distribution of planetary nebulae with warm dust emission features - I

We investigate the Galactic disc distribution of a sample of planetary nebu lae characterized in terms of their mid-infrared spectral features. The tot al number of Galactic disc PNe with 8-13 mum spectra is brought up to 74 wi th the inclusion of 24 new objects, the spectra of which we present for the first time. 54 PNe have clearly identified warm dust emission features, an d form a sample that we use to construct the distribution of the C/O chemic al balance in Galactic disc PNe. The dust emission features complement the information on the progenitor masses brought by the gas-phase N/O ratios: P Ne with unidentified infrared emission bands have the highest N/O ratios, w hile PNe with the silicate signature have either very high N enrichment or close to none. We find a trend for a decreasing proportion of O-rich PNe to wards the third and fourth Galactic quadrants. Two independent distance sca les confirm that the proportion of O-rich PNe decreases from 30 +/- 9 per c ent inside the solar circle to 14 +/- 7 per cent outside. PNe with warm dus t are also the youngest. PNe with no warm dust are uniformly distributed in C/O and N/O ratios, and do not appear to be confined to C/O similar to 1. They also have higher 6-cm fluxes, as expected from more evolved PNe. We sh ow that the IRAS fluxes are a good representation of the bolometric flux fo r compact and TR-bright PNe, which are probably optically thick. Selection of objects with F(12 mum) > 0.5 Jy should probe a good portion of the Galac tic disc for these young, dense and compact nebulae, and the dominant selec tion effects are rooted in the PN catalogues.