H2 in low-ionization structures of planetary nebulae
Abstract
We report the detection of near-IR H2 emission from the low-ionization structures (knots) in two planetary nebulae. The deepest ever, high-angular resolution H2 1-0 S(1) at 2.122μ, H2 2-1 S(1) at 2.248μ and Brγ images of K 4-47 and NGC 7662, obtained using the Near InfraRed Imager and Spectrometer (NIRI) at Gemini-North, are analyzed here. K 4-47 reveals a remarkable highly collimated bipolar structure not only in the optical but also in the molecular hydrogen emission. The H2 emission emanates from the walls of the bipolar outflows and also from the pair of knots at the tip of the outflows. The H2 1-0 S(1)/2-1 S(1) line ratio ranges from ~7 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 H2 line, v=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 H2 v=2-1 S(1) line. These four knots exhibit an H2 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 confirms the presence of H2 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 share similar traits to photodissociation regions.
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