H2 in low-ionization structures of planetary nebulae

We report the detection of near-IR H-2 emission from the low-ionization structures (knots) in two planetary nebulae. The deepest ever high-angular-resolution H-2 (1-0) S(1) at 2.122 mu m, H-2 (2-1) S(1) at 2.248 mu m and Br gamma images of K 4-47 and NGC 7662, obtained using the Near InfraRed Imager and Spectrometer (NIRI) at Gemini-North, are analysed here. K 4-47 reveals a remarkable highly collimated bipolar structure not only in the optical but also in the molecular hydrogen emission. The H-2 emission emanates from the walls of the bipolar outflows and also from the pair of knots at the tip of the outflows. The H-2 (1-0) S(1)/(2-1) S(1) line ratio ranges from similar to 7 to similar to 10, suggesting the presence of shock interactions. Our findings can be explained by the interaction of a jet/bullet ejected from the central star with the surrounding asymptotic giant branch material. The strongest H-2 line, (1-0) S(1), is also detected in several low-ionization knots located at the periphery of the elliptical planetary nebula NGC 7662, but only four of these knots are detected in the H-2 (2-1) S(1) line. These four knots exhibit an H-2 line ratio between 2 and 3.5, which suggests that the emission is caused by the UV ionizing flux of the central star. Our data confirm the presence of H-2 gas in both fast-and slow-moving low-ionization knots, which has only been confirmed before in the nearby Helix nebula and Hu 1-2. Overall, the low-ionization structures of planetary nebulae are found to have similar traits to photodissociation regions.