Fluorescent molecular hydrogen in the Eagle nebula

Details Fluorescent molecular hydrogen in the Eagle nebula Fluorescent molecular hydrogen in the Eagle nebula

Mar 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999mnras.304...98a&link_type=abstract

Monthly Notices of the Royal Astronomical Society, Volume 304, Issue 1, pp. 98-108.

Astronomy and Astrophysics

Astronomy

15

Shock Waves, Ism: Clouds, Ism: Individual: M16, Ism: Molecules, Infrared: Ism: Lines And Bands

Scientific paper

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 H_2 1-0 S(1) and 2-1 S(1) lines at 2.122 and 2.248 mu m, respectively, and in the H i Br gamma line at 2.166 mu m. The trunk-like features have an average H_2 number density of ~ 10^4 cm^-3 and are irradiated by a far-UV field &sim 10^4 x the ambient interstellar value. The H_2 intensity profile 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 find that most of the H_2 emission from the source is consistent with purely fluorescent excitation, however a significant fraction of the H_2 emission (~ 25 per cent) from the northernmost column shows evidence for `collisional fluorescence', i.e. redistribution of H_2 level populations through collisions. This emission is confined to clumps up to ~ 0.01 pc in diameter, with densities >= 10^5 cm^-3, and perhaps > 10^6 cm^-3, filling at most a few per cent of the volume of the trunks. The line intensities and ratios are consistent with steady-state and not time-dependent PDR models.

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