High-resolution near-infrared spectroscopy of Hubble

Spectra at H (1.6 μm) and K (2.2 μm) have been obtained of the young planetary nebula Hubble 12. The prominent features in these rich spectra include emission from the Brackett and Pfund series of hydrogen, [Fe II], [Fe III], Mg II, He I, and H₂. Our high-resolution spectra (R_H ∼ 2200, R_K ∼ 3500) have revealed ∼50 H₂ lines at both H and K. H₂ emission in the H window and transitions from levels as high as v = 12 confirm the fluorescent excitation. The large number of detected H₂ lines allows us to compare in detail the observed strengths to those predicted by models of fluorescent emission. We find good agreement with models that give gas densities in the outer regions of the nebula close to n_H = 3.0 × 10³ cm^-3. The surface brightness of the 1-0 S(1) line is an order of magnitude higher than that predicted by models of steady state fluorescent emission, which is possibly due to the time dependence of the emission. The helium emission lines are found to agree well with theoretical predictions of helium recombination, although the blended lines at 2.11 μm imply a rather high T_e ∼ 2 × 10⁴ K. The line ratios of seven [Fe II] and three [Fe III] emission features in the H and K windows, respectively, are found to agree with the latest theoretical predictions for T_e ∼ 10⁴ K and n_e ∼ 10⁴ cm^-3. This emission may originate in pockets of shocked material in the core of the nebula. The hydrogen emission in both the Brackett and Pfund series is found to differ significantly from case B recombination predictions, the cause of which is as yet unexplained.