Improved abundances in three halo planetary nebulae.

Spectrophotometric observations have been obtained of three halo planetary nebulae: 49+88.1 deg, 108-76.1 deg, and K648. The observed line intensities are used to compute temperatures, densities, and abundances of helium, oxygen, nitrogen, and neon with respect to hydrogen. The data show that for all three nebulae, the helium abundance is normal, but oxygen is underabundant with respect to disk planetaries. That the abundance of nitrogen varies from object to object suggests that mixing in differing amounts may have taken place. In 49+88.1 deg, the only halo planetary with detectable forbidden S II or S III emission, sulfur is underabundant by a factor of about 10.