2020MNRAS.493.3800A

Despite the many studies in the last decades, the low-ionization structures (LISs) of planetary nebulae (PNe) still hold several mysteries. Recent imaging surveys have demonstrated that LISs are composed of molecular gas. Here, we report H₂ emission in the LISs of NGC 7009 and NGC 6543 by means of very deep narrow-band H₂ images taken with The surface brightness of the H₂ 1-0 S(1) line is estimated to be (0.46-2.9)x 10^–4 erg s^–1 cm^–2 sr^–1 in NGC 7009 and (0.29-0.48)x 10^–4 erg s^–1 cm^–2 sr^–1 in NGC 6543, with signal-to-noise ratios of 10-42 and 3-4, respectively. These findings provide further confirmation of hidden H₂ gas in LISs. The emission is discussed in terms of the recent proposed diagnostic diagram R(H₂) = H₂ 1-0 S(1)/H₂ 2-1 S(1) versus R(Brγ) = H₂ 1-0 S(1)/Brγ, which was suggested to trace the mechanism responsible for the H₂ excitation. Comparing our observations to shock and ultraviolet (UV) molecular excitation models, as well as a number of observations compiled from the literature showed that we cannot conclude for either UV or shocks as the mechanism behind the molecular emission.