H${}_2$ emission in the low-ionisation structures of the Planetary Nebulae NGC 7009 and NGC 6543

Abstract

Despite the many studies in the last decades, the low-ionisation 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 H${}_2$ emission in the LISs of NGC 7009 and NGC 6543 by means of very deep narrow-band H${}_2$ images taken with NIRI@Gemini. The surface brightness of the H2 1-0 S(1) line is estimated to be (0.46-2.9)$\times$10${}^{-4}$ erg s${}^{-1}$ cm${}^{-2}$ sr${}^{-1}$ in NGC 7009 and (0.29-0.48)$\times$10${}^{-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 H${}_2$ gas in LISs. The emission is discussed in terms of the recent proposed diagnostic diagram R(H${}_2$)=H${}_2$ 1-0 S(1)/H${}_2$ 2-1 S(1) versus R(Br$\gamma$)=H${}_2$ 1-0 S(1)/Br$\gamma$, which was suggested to trace the mechanism responsible for the H${}_2$ 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.

11 pages, 6 figures, 2 tables, accepted for Publication in MNRAS

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