tokens: data to analyse them together with our CO ( J = 3−2) observations. BD+30 3639 was the source with the best complementary CO ( J = 2−1) data, given the size of the source and the beam size of the CO ( J = 2−1) observations. A discussion of the use of these diagnostic plots is presented for different molecules in the Appendix of van der Tak et al. (2007) . Linear molecules, such as CO, are density probes at low densities, while at higher densities, the line ratios are more sensitive to temperature. However, with no other information than a single line ratio (such as our case), it is only possible to provide lower limits to density and temperature. Table 4 Size, distance, and morphology of PNe with CO emission. References. (1) Cahn et al. (1992) , (2) Robinson et al. (1992) , (3) Groenewegen et al. (2012) , (4) Frew (2008) , (5) Sahai et al. (2007) , (6) Acker et al. (1992) , (7) Tajitsu Tamura (1998) , and (8) Guzmán et al. (2006) . In Fig. 2 we show the diagnostic plot for the CO ( J = 3−2) to CO ( J = 2−1) intensity ratio, assuming 2.73 K as the background temperature and a line width of 50 km s −1 . The plot shows the results for a column density of 1 × 10 16 cm −2 , but we find that the CO line ratio is not very sensitive to column density within the range 3 × 10 15 cm −2 to ~4 × 10 16 cm −2 . The plotted ratios corresponded to the observed values for BD+30 3639 (1.6), NGC 3132 (1.3), IC 4406, and M2-9 (0.7, for the last two). However, for the latter three sources, the measured ratios are only lower limits because of different CO ( J = 2−1) and CO ( J = 3−2) beam sizes (the former larger than the latter) and the source size (larger than both beam sizes). The CO intensity ratio constrains a lower limit to the H 2 volume density, n H 2 > 10 3 –10 3.7 cm −3 , and kinetic temperature, T kin > 10–35 K. The highest lower limit values are given for IC 4406 and M2-9. 3.6 Correlation with H<sub>2</sub> We would like to probe the relationship between the detection of CO and H 2 . We compared our sample with the sample analysed by Kastner et al. (1996) , whose study was made by imaging the molecular hydrogen emission ( λ = 2.122 μ m) using the near-infrared cryogenic optical bench mounted at Kitt Peak National Observatory, from the National Optical Astronomy Observatories. They observed 60 PNe and detected H 2 in 23 of them. When compared with out sample, 9 objects are in common. Of these 9 objects, 4 were detected in H 2 , and all are also detected in CO as part of our survey. The objects are NGC 3132, IC 4406, PN M2-9, and BD+30 3639. The remaining objects (five) were not detected in H 2 and are they detected here in CO either. These PNe are PN M3-2, PN MyCn18, NGC 6778, NGC 6790, and HD 193538. Although the overlap between PANDORA and the H 2 survey of Kastner et al. (1996) is small, we find a clear correlation between the detection of CO and the detection of H 2 . Fig. 2 Results from the LVG analysis of the CO lines. The curves represent the
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