The excitation mechanism of H2 in bipolar planetary nebulae

We present near-IR K-band intermediate-dispersion spatially-resolved spectroscopic observations of a limited sample of bipolar planetary nebulae (PNe). The spectra have been used to determine the excitation mechanism of the H-2 molecule using standard line ratios diagnostics. The H-2 molecule is predominantly shock-excited in bipolar PNe with broad equatorial rings, whereas bipolar PNe with narrow equatorial waists present either UV excitation at their cores (e. g. Hb 12) or shock-excitation at their bipolar lobes (e. g. M1-92). The shock-excitation among bipolar PNe with ring is found to be correlated with emission in the H-2 1-0 S(1) line brighter than Br gamma. We have extended this investigation to other PNe with available near-IR spectroscopic observations. This confirms that bipolar PNe with equatorial rings are in average brighter in H-2 than in Br gamma and show dominant shock-excitation.