H2 and Br-gamma narrow-band imaging of bipolar planetary nebulae

We present near-IR narrow-band continuum-subtracted images in the H${}_2$ $2.122\mu$m, and Br$\gamma$ $2.166\mu$m emission lines for a sample of 15 bipolar planetary nebulae. H${}_2$ emission was definitely detected for most of the objects in this sample (13 out of 15). The very high H${}_2$ detection rate supports the idea that bipolar planetary nebulae have important reservoirs of molecular material and offer suitable physical conditions for the excitation of H${}_2$. The strength of the H${}_2$ emission and the H${}_2$/Br$\gamma$ flux ratio are found to correlate with the morphology of the bipolar nebulae observed. Bipolar PNe with broad and bright rings exhibit stronger H${}_2$ emission than bipolar PNe with narrow twists. High-quality (sub-arcsec) [N~{\sc ii}] and H$\alpha$ optical images have been used to compare the distribution of the ionized and molecular material. The H${}_2$ emission lies just outside the optical [N~{\sc ii}] emission zone.