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Summary: Fluorescent molecular hydrogen in the Eagle nebula
L. E. Allen,* M. G. Burton, S. D. Ryder,² M. C. B. Ashley and J. W. V. Storey
School of Physics, University of New South Wales, Sydney 2052, Australia
Accepted 1998 November 11. Received 1998 November 6; in original form 1998 March 25
A B S T R AC T
We used the University of New South Wales Infrared Fabry-Perot (UNSWIRF) to
investigate the photodissociation region (PDR) associated with the `elephant trunk'
features in the M16 H ii region (the Eagle nebula). Images were made in the H2 1±0
S(1) and 2±1 S(1) lines at 2.122 and 2.248 mm, respectively, and in the H i Br g line at
2.166 mm. The trunk-like features have an average H2 number density of ,104
cmÀ3
and
are irradiated by a far-UV ®eld ,104
´ the ambient interstellar value. The H2 intensity
pro®le across the trunks is consistent with a simple model in which cylindrical columns of
gas are illuminated externally, primarily by a direct component (the stars of NGC 6611),
with an additional contribution from an isotropic component (scattered light). We ®nd that
most of the H2 emission from the source is consistent with purely ¯uorescent excitation,
however a signi®cant fraction of the H2 emission (,25 per cent) from the northernmost
column shows evidence for `collisional ¯uorescence', i.e. redistribution of H2 level
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