Planetary nebulae

Abstract

A large number of infrared fine structure lines have now been observed from planetary nebulae and they provide a powerful tool for the determination of nebular physical parameters and abundances, for both ionised and neutral regions. H2 line emission has been detected from 50% of the PN so far surveyed and, rather surprisingly, shows a higher probability of being detected around old, low-density nebulae than around young, dense nebulae. Photodissociation zone models seem promising in some cases. Spectroscopy at 10μm shows that of the nebulae with detectable continua, 33% show silicate features, 25% show SiC features and 42% show strong UIR bands. These feature types follow a sequence of increasing nebular C/O ratio. A number of carbon-rich PN have also been found to exhibit a very strong 30μm feature which has been attributed to MgS. Recent work on the modelling of the dust emission continua and on the derivation of gas-to-dust mass ratios is also reviewed. -Author