Planetary nebulae expansion distances .3.

In a continuing project to determine accurate expansion-parallax distances to planetary nebulae, we have performed high-quality lambda 6-cm continuum observations of 12 nebulae with the Very Large Array. All of the sources in this imaging survey have one epoch of lambda 6-cm continuum data in the VLA archives. With two epochs for each planetary nebula, we applied the expansion parallax algorithm. Due in large part to the poor quality of the preexisting epoch, we were successful in only one case (NGC 7662), and obtained a lower limit to the distance for two PNe (Hu 1-2 and NGC 2440). Here, we also present new images acquired in this survey with the intent of reobserving the sample. Similar observations of these nebulae planned in 1997 should be sufficient to detect the expansion of a large percentage of the nebular shells.