Abstract
It has recently been noted that the kinematic parallaxes of planetary nebulae (PNe) may be significantly in error. The pattern velocities of the shells, which determine the lateral expansion of the outflows, are likely to differ from line-of-sight velocities determined through visual spectroscopy. It is usually assumed that the mean distances to large ensembles of PNe should be reasonably secure, however. Given that sources are randomly oriented with respect to the line-of-sight, then individual distance biases should wash-out in the mean. We point out that this is unlikely to be the case where sources possess non-spherical structures, however. If one measures lateral expansion velocities Vbot along the minimum axes of the sources, then distances will (in the mean) tend to be somewhat too large. Alternatively, if one consistently determines Vbot along the largest axes of the outflows, then values of < D > will tend to be low. Although the size of error is difficult to assess, it may approach of order Δ D/D∼ 10%.
