1999A&A...342..823V

This paper presents new near-infrared observations of the planetary nebula NGC 2346. The data include a broad K-band image, an image in the H₂ vibrationally excited 1-0S(1) line and K band slit spectra at three positions in the nebula. In the H₂ 1-0S(1) line, the nebula is characterized by a central, bright torus, surrounded by weaker emission with a typical butterfly shape, as seen in Hα and CO lines. The K band spectra show 11 H₂ lines with excitation energies from 6150 to 12552K. The H₂ data have been compared to the predictions of models which follow the evolution with time of the H₂ emission in PNe of different core mass and shell properties (Natta & Hollenbach, 1998A&A...337..517N). These models compute the emission originating in the photodissociation region (PDR) created at the inner edge of the neutral shell by the UV radiation of the central core, as well as the emission in the shock associated with the expansion of the shell inside the precursor red-giant wind. In NGC 2346, a PDR origin of the H₂ emission in a low-density molecular shell (n≲10⁴cm^–3) is indicated. At these low densities, time-dependent H₂ chemistry and X-ray heating of the neutral gas enhance the predicted PDR H₂ line intensity by large factors.