Astronomy and Astrophysics – Astrophysics
Scientific paper
May 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995a%26a...297..246s&link_type=abstract
Astronomy and Astrophysics (ISSN 0004-6361), vol. 297, no. 1, p. 246-250
Astronomy and Astrophysics
Astrophysics
11
Hydrogen Ions, Infrared Imagery, Ion Emission, Line Spectra, Molecular Ions, Molecular Spectra, Planetary Nebulae, Infrared Spectra, Interstellar Matter, Photodissociation, Stellar Luminosity, Stellar Spectra
Scientific paper
An infrared image of the planetary nebula NGC 2818 in the H2 1-0S(1) transition is presented. The H2 emission originates from an almost complete inner elliptical ring-like structure and an outer southern lobe. The inner ring has extensions of 35 sec x 60 sec and a width of typically approximately 8 sec which corresponds to a projected thickness of 0.1 pc. The most intense H2 emission comes from the outer southern lobe which is separated from the inner shell by about 10 sec. The total energy emitted by NGC 2818 at 2.12 micron is approximately 1 solar luminosity which amounts to approximately 1% of the luminosity of the central star. The strongest H2 radiation lies close to the H(+)-H(0) transition zone, but weaker H2 emission can also be detected well inside the ionized area. In some parts of the nebulosity, the H2 emission closely traces the H-alpha emission pattern. In these areas, the H2 molecule apparently is able to survive in neutral pockets with sizes less than approximately 1016 cm. Comparisons with models of Photo-Dissociation Regions (PDRs) shows that fluorescence can not account for the observed H2 intensities. The low level emission in the inner H2 shell can be generated by thermal excitation following UV pumping. The strong H2 emission peak in the southern lobe is probably due to shock excitation.
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