Flux ratio of HNC to HCN plotted against PN central star UV luminosity. Line ratios obtained from observations by Schmidt & Ziurys (2017a) are denoted by crosses. CSPN UV luminosities from Table 1 and Bachiller et al. (1989b, NGC 2346), Frew (2008, NGC 2440 and Hb 5), and Sabbadin et al. (1984, M 1-7). The best-fit linear power-law slope (black dashed line, NGC 7027 and literature sources omitted) is m = − 0.363 with a linear correlation coefficient of r = − 0.885. Typical uncertainty estimate of 40% for UV luminosity and 10% for line ratios is plotted in the top right.