IR colours of MASH planetary nebulae

We were able to identify ~5 per cent of the sources in the NIR, and a slightly larger fraction (~12 per cent) in the FIR. It is concluded that whilst the NIR colours of these nebulae are consistent with those of less evolved (and higher surface brightness) PNe, their FIR colours are markedly different.

This disparity is likely to arise as a result of an evolution in dust temperatures, in their line emission characteristics, and in the relative contributions of the 8.6 and 11.3um polycyclic aromatic hydrocarbon emission features. A rump of~9 per cent of the detected sources have values log[F(25um)/F(60um)] which are lower than can be explained in terms of normal nebular evolution, however. If these are comparable in nature to the undetected PNe, then this would argue that ~1 in 10 of MASH I and II nebulae may represent galactic H ii regions, Stromgren spheres, symbiotic nebulae or other unrelated categories of source.