Millimetre and submillimetre continuum observations of planetary nebulae

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Abstract
Continuum emission from a total of 13 planetary nebulae (PN) has been detected at millimetre and submillimetre wavelengths, in all but one case for the first time. It was found that Type I and young, compact PN both have significant dust emission at 450 and 800 μm, while at 1100 and 2000 μm the free–free emission from the nebular gas dominates. More evolved PN do not usually show any cool dust excess above the free–free emission at 800 μm. We have also spatially resolved the cool dust in the neutral regions surrounding NGC 7027 and BD + 30°3639 for the first time. In NGC 7027 the 450-μm emission seems to correlate spatially with the optical extinction in front of the nebula and the CO emission. We model the IR submillimetre observations for NGC 7027 and BD + 30°3639 using amorphous carbon dust; graphite not being able to provide enough submillimetre flux. In the ionized regions of these two young, compact nebulae we deduce that the dust-to-gas ratios are no higher than about 7 × 10−4. This is similar to large older PN and, therefore, there is no evidence here that the dust content drops as PN evolve. The dust-to-gas ratio in the neutral region around NGC 7027 is about 1.5 × 10−3, which means that dust is not significantly destroyed in the ionized region of the nebula.

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