Molecular and Atomic Excitation Stratification in the Outflow of the Planetary Nebula M27

High resolution spectroscopy with FUSE and STIS of atomic and molecularvelocity stratification in the nebular outflow of M27 challenge models for theabundance kinematics in planetary nebulae. The simple picture of a very highspeed (~ 1000 km/s), high ionization, radiation driven stellar wind surroundedby a slower (~ 10 km/s) mostly molecular outflow, with low ionization andneutral atomic species residing at the wind interaction interface, is notsupported... We find ...there is a fast (33 -- 65 km/s) low ionization zone,surrounding a slower (<~ 33 km/s) high ionization zone and, at the transitionvelocity (33 km/s), vibrationally excited H_2 is intermixed with apredominately neutral atomic medium... Far-UV continuum fluorescence of H_2 isnot detected, but Lyman alpha (Lya) fluorescence is present. The diffusenebular medium is inhospitable to molecules and dust. Maintaining the modestequilibrium abundance of H_2 (N(H_2)/N(HI) << 1) in the diffuse nebular mediumrequires a source of H_2, mostly likely the clumpy nebular medium. The stellarSED shows no signs of reddening (E(B-V) < 0.01), but paradoxically measurementsof Ha/Hb ... indicate E(B-V) ~ 0.1. ...the apparent enhancement of Ha/Hb in theabsence of dust may result from a two step process of H_2 ionization by Lymancontinuum (Lyc) photons followed by dissociative recombination (H_2 + gamma ->H_2^+ + e -> H(1s) + H (nl)), which ultimately produces fluorescence of Ha andLya. In the optically thin limit at the inferred radius of the velocitytransition we find dissociation of H_2 by stellar Lyc photons is an order ofmagnitude more efficient than spontaneous dissociation by far-UV photons. Wesuggest that the importance of this H_2 destruction process in HII regions hasbeen overlooked.Comment: In emulateapj 2 column, 28 pages total, 18 figures. Accepted for publication in the Astrophysical Journal on 5 January 2007. Abstract abridge