METAL-POOR PLANETARY-NEBULAE - EFFECTS OF WINDS

Some planetary nebulae in the galactic thick disk display extremely low abundances of heavy elements such as O, Ne, S, and Ar, compared with ''normal'' or type II nebulae. Their central stars are generally relatively cool and underluminous, indicating that the progenitor stars had very low masses. It is suggested that strong stellar winds have had an important role in the formation of these objects, which is supported by the large mass loss rates now observed.