ON AN ALTERNATIVE STATISTICAL DISTANCE SCALE FOR PLANETARY-NEBULAE

We propose to use the correlation between the distance-independent rad io continuum brightness temperature and the distance-dependent radius to determine statistical distances to planetary nebulae. This correlat ion satisfies two objective criteria which define a statistical distan ce scale: (1) We obtain the same calibration relation for the large sa mple of Galactic bulge planetary nebulae as for a small sample of non- bulge planetary nebulae with well-determined distances, showing that t he method is applicable to different PN populations. (2) The distribut ion of the distances to the Galactic bulge planetary nebulae is in agr eement with the expected distribution of distances around the Galactic center. Distances could be determined to better than a factor 2 for 9 5% of the planetary nebulae, and on average to an accuracy of about 40 %. It is an essential requirement that the optically thin radio flux a nd corresponding angular diameter be used in calculation of the bright ness temperature. We show that the correlation is also reproduced by m odel calculations of PN evolution. We show that the new calibration of the Daub scale obtained by Cahn et al. (1992) also gives the correct average distance to the large sample of bulge planetary nebulae, but g ives a skewed distribution around the Galactic Center. Models show tha t this is caused by the fact that for young nebulae the Daub curve doe s not follow evolutionary model tracks.