2007ApJ...659.1291M

High-resolution spectroscopy with FUSE and HST STIS of atomic and molecular velocity stratification in the nebular outflow of M27 challenge models of the abundance kinematics in planetary nebulae. The simple picture of a very high speed (∼1000 km/s), high-ionization, radiation-driven stellar wind surrounded by a slower (∼10 km/s) mostly molecular outflow, with low-ionization and neutral atomic species residing at the wind interaction interface, is not supported. Instead, we find vibrationally excited H₂ intermixed with mostly neutral atomic species at a transition velocity (33 km/s) between a fast (33-65 km/s) low-ionization zone and a slow (≲33 km/s) high-ionization zone. Lyα fluorescence of H₂has been detected, but far-UV continuum fluorescence has not. The diffuse nebular medium is inhospitable to molecules and dust. Maintaining a modest equilibrium abundance of H₂[N(H₂)/N(H I)≪1] in the diffuse nebular medium requires a source of H₂, mostly likely the clumpy nebular medium. The stellar SED shows no sign of reddening [E(B-V)<0.01], but paradoxically Hα/Hβ indicates E(B-V)∼0.1. The enhancement of Hα/Hβ in the absence of dust may result from a two-step process of H₂ ionization by Lyman continuum photons followed by dissociative recombination [H₂+ γ ⟶ H⁺₂ + e^– ⟶ H(1s) + H(nl)], which ultimately produces fluorescence of Hα and Lyα. In the optically thin limit at the inferred radius of the velocity transition, we find that dissociation of H₂ by stellar Lyman continuum photons is an order of magnitude more efficient than spontaneous dissociation by far-UV photons. The importance of this H₂ destruction process in H II regions has been overlooked.