Planetary nebula distances re-examined: an improved statistical scale

The distances of planetary nebulae (PNe) are still quite uncertain. Although observational estimates are available for a small proportion of PNe, based on statistical parallax and the like, such distances are very poorly determined for the majority of galactic PNe. In particular, estimates of so-called `statistical' distance appear to differ by factors of ~2.7.

We point out that there is a well-defined correlation between the 5-GHz luminosity of the sources, L₅, and their brightness temperatures, T_B. This represents a different trend to those investigated in previous statistical analyses, and permits us to determine independent distances to a further 449 outflows. These distances are shown to be closely comparable to those determined using a T_B-R correlation, providing that the latter trend is taken to be non-linear.

This non-linearity in the T_B-R plane has not been noted in previous analyses, and is likely responsible for the broad (and conflicting) ranges of distance that have previously been published.

Finally, we point out that there is a close accord between observed trends within the L₅-T_B and T_B-R planes, and the variation predicted through nebular evolutionary modelling. This is used to suggest that observational biases are probably modest, and that our revised distance scale is reasonably trustworthy.