A LOWER LIMIT ON THE DISTANCE TO THE YOUNG PLANETARY-NEBULA VY 2-2 BY EXPANSION PARALLAX

We report and discuss new high resolution radio continuum and H76-alpha recombination line observations of the young compact planetary nebula Vy 2-2. The continuum images are compared with similar images made approximately five years earlier to search for evidence of expansion. No expansion was detected, and the upper limit on the expansion rate is combined with the expansion velocity obtained from the line observations to deduce a lower limit of 5 kpc on the distance to Vy 2-2. The expansion velocity derived in this paper is compared with other results in the literature, primarily in the optical, to propose a simple picture for the differences observed. This analysis is important for obtaining a reliable estimate of the distance by the expansion parallax method. It is suggested that there are at least two separate regimes in velocity, and that the observed expansion velocity depends upon the density in the shell where the line is formed. The different regimes are produced by the interaction between the fast wind and then the molecular envelope of the planetary nebula.