SIMBAD references

We point out that many planetary nebulae (PNe) have large infrared indices (H-K_S), and that this is likely to result from the presence of hot grains, and/or H₂S(1) line emission at λ= 2.122µm. We are able to identify two groups of sources associated with each of these mechanisms, both of which appear to possess distinct physical characteristics. One difference between these groups concerns the near-infrared dimensions of the sources. It appears that hot dust outflows frequently have sizes θ(K_S) > θ(H) > θ(J). Four of the sources are particularly extreme in this regard, and show emission extending well outside of the primary shells. We propose that this is likely to arise from high-temperature grains located in low-density haloes. The location of such grains at large distances from the central stars represents a challenge for any mechanism purporting to explain this phenomenon. The most likely explanation appears to be in terms of photon heating of very small grains.