1995A&A...297..246S

An infrared image of the planetary nebula NGC 2818 in the H₂ 1-0 S(1) transition is presented. The H₂ emission originates from an almost complete inner elliptical ring-like structure and an outer southern lobe. The inner ring has extensions of 35"x60" and a width of typically ∼8" which corresponds to a projected thickness of 0.1pc. The most intense H₂ emission comes from the outer southern lobe which is separated from the inner shell by about 10". The total energy emitted by NGC 2818 at 2.12µm is ∼1L_☉ which amounts to ∼1% of the luminosity of the central star. The strongest H₂ radiation lies close to the H⁺-H⁰ transition zone, but weaker H₂ emission can also be detected well inside the ionized area. In some parts of the nebulosity, the H₂ emission closely traces the Hα emission pattern. In these areas, the H₂ molecule apparently is able to survive in neutral pockets with sizes ≲10¹⁶cm. Comparison with models of PDR's shows that fluorescence can not account for the observed H₂ intensities. The low level emission in the inner H₂ shell can be generated by thermal excitation following UV pumping. The strong H₂ emission peak in the southern lobe is probably due to shock excitation.