The spectrum of NGC 7027 from 3080 to 2630 wavenumbers (3.25-3.80 microns) - Detection of new atomic and molecular hydrogen lines and new constraints on the chemical sidegroups on polycyclic aromatic hydrocarbons

Abstract

An infrared spectrum of NGC 7027, with a resolving power of 89 930 over the 279 2690 cm<SUP>-1</SUP> (3.58 3.72 μm) range, is presented which shows several emission lines due to atomic and molecular hydrogen. Three of the lines fall near 2773, 2743, and 2710 cm<SUP>-1</SUP> (3.606, 3.645, and 3.690 μm) and are identified with the (20 → 6), (19 → 6), and (18 → 6) Humphreys series transitions of atomic hydrogen, respectively. The strengths of these lines are consistent with those expected from the observed intensity of the Pfund-γ (8 → 5) line in this object. The fourth line falls near 2760 cm<SUP>-1</SUP> (3.623 μm) and is tentatively identified with the (0 → 0) S(15) transition of molecular hydrogen. It is not presently clear whether the strength of the newly observed (0 → 0) S(15) H<SUB>2</SUB> line relative to the previously detected H<SUB>2</SUB> lines in this object is consistent with shock excitation or fluorescence from clumpy photodissociation regions. <P />It is difficult to reconcile the relative strengths of the 3040, 2940, 2890, and 2850 cm<SUP>-1</SUP> (3.29, 3.40, 3.46, and 3.51 μm) emission features in the spectrum of NGC 7027 with an origin in aliphatic sidegroups on polycyclic aromatic hydrocarbons (PAHs). The inconsistencies in the sidegroup model are minimized (but not eliminated) if the sidegroups are assumed to consist predominantly of methyl (-CH<SUB>3</SUB>) groups. The absence of an emission feature centered near 2740 cm<SUP>-1</SUP> (3.65 μm), which is characteristic of methyl groups attached to aromatics, places a severe upper limit on the number of methyl sidegroups that can be present in this object. If methyl groups are present at all, this upper limit implies that the PAH population in NGC 7027 must contain at least 400 peripheral aromatic C-H bonds for every peripheral methyl group. This -CH<SUB>3</SUB> abundance is a factor of 2-12 times too low to explain the strengths of the 2940, 2890, and 2850 cm<SUP>-1</SUP>(3.40, 3.46, and 3.51 μm) features in the context of aliphatic sidegroups. Together these inconsistencies strongly indicate that the 2940, 2890, and 2850 cm<SUP>-1</SUP> (3.40, 3.46, and 3.51 μm) interstellar emission features are not predominantly due to aliphatic sidegroups on PAHs