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 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 H _ { 2 } 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 can not conclude for either UV or shocks as the mechanism behind the molecular emission .