Infrared photometry of southern planetary nebulae and emission-line objects

Ten-micron photometry is presented of 23 planetary nebulae, seven very low excitation (VLE) nebulae, and 11 emission-line objects, of which more than half were also detected at 20 microns. The 10-micron behaviors of the various classes of objects are compared, and it is shown that planetary nebulae can easily be distinguished from VLE nebulae by virtue of the silicate emission displayed by the latter. A uniform sample of planetary nebulae for statistical analysis is created by combining the present sample with an earlier northern hemisphere survey. It is shown that L alpha photons are the dominant grain heating mechanism for nebular electron densities less than approximately 15,000/cu cm, whereas above this density direct dust absorption of stellar continuum photons dominates