The Kinematics and Excitation of Molecular Hydrogen Emission in the Planetary Nebula BD +30°3639

High spatial and spectral resolution spectra of the near-infrared H₂ emission from the planetary nebula BD +30°3639 are presented. A 7'' × 13'' portion of the nebula was mapped at a resolution of 6000 in the v = 1-0 S(1) emission line of molecular hydrogen, revealing the morphology and kinematics of the H₂ emission. These spectra show large velocity shifts in the H₂ line across the nebula. Comparison with single-dish CO observations show significant velocity differences between the peaks of H₂ emission and CO emission. Such differences are unexpected for a photodissociation region (PDR), since PDRs are supposed to form in the surface of the cold gas (as is the case for NGC 7027).

A single long-slit spectrum which includes the peak of the H₂ emission and covers the entire K band at a resolution of 1000 is analyzed. Fourteen molecular hydrogen emission lines are detected in a 2'' × 05 region covering the bright northeastern H₂ lobe. The line ratios indicate that the H₂ emission is radiatively excited, but that the gas density is large enough (n ≳ 10^4.5-10⁵ cm^-3) for collisions to thermalize the lowest vibrational states. The measured ratio of ortho-hydrogen to para-hydrogen is 2.51 ± 0.22, slightly smaller than the equilibrium value of 3. Spatial variations of the v = 2-1 vibrational temperature and the v = 1 ortho/para ratio are consistent with a decrease in the relative proportion of collisional excitation as distance from the central star increases.