A near-infrared study of the planetary nebula NGC 2346

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 H-2 vibrat ionally excited 1-0S(1) line and K band slit spectra at three positions in the nebula. in the H-2 1-0S(1) line, the nebula is characterized by a centr al, bright torus, surrounded by weaker emission with a typical butterfly sh ape, as seen in Her and CO lines. The K band spectra show II H-2 lines with excitation energies from 6150 to 12552 K. The H-2 data have been compared to the predictions of models which follow the evolution with time of the H- 2 emission in PNe of different core mass and shell properties (Natta & Holl enbach 1998). These models compute the emission originating in the photodis sociation region (PDR) created at the inner edge of the neutral shell by th e UV radiation of the central core, as well as the emission in the shock as sociated with the expansion of the shell inside the precursor red-giant win d. In NGC 2346, a PDR origin of the H-2 emission in a low-density molecular shell (n less than or similar to 10(4) cm(-3)) is indicated. At these low densities, time-dependent H-2 chemistry and X-ray heating of the neutral ga s enhance the predicted PDR H-2 line intensity by large factors.