H₂in low-ionization structures of planetary nebulae

Abstract

We report the detection of near-IR H<inf>2</inf> emission from the low-ionization structures (knots) in two planetary nebulae. The deepest ever high-angular-resolution H<inf>2</inf> (1–0) S(1) at 2.122 μm, H<inf>2</inf> (2–1) S(1) at 2.248 μm and Brγ 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<inf>2</inf> emission emanates from the walls of the bipolar outflows and also from the pair of knots at the tip of the outflows. The H<inf>2</inf> (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 H<inf>2</inf> 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<inf>2</inf> (2–1) S(1) line. These four knots exhibit an H<inf>2</inf> 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<inf>2</inf> 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.