Distances to planetary nebulae. II. HI absorption observations.

Measurements of 21-cm absorption spectra for 24 PN have been carried out, together with the 21-cm absorption spectrum of a background source in each case. The 21-cm emission spectra in the direction towards the PN have also been measured. Kinematic distances to the PN are either derived using a flat galactic rotation curve or using distances and radial velocities of H II regions to determine the galactic structure. For 12 planetary nebulae, the final distance derived has an uncertainty of less than 50 percent, a significant improvement over previous distance determinations. H I, which is thought to be associated with the PN, was found only in the absorption spectrum of NGC 6790. A rough estimate of the lower limit to the total mass of the H I involved gives 0.01 solar mass.