Physical conditions in the transition regions around the Ring Nebula and NGC 7027

Details Physical conditions in the transition regions around the Ring Nebula and NGC 7027 Physical conditions in the transition regions around the Ring Nebula and NGC 7027

Mar 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996mnras.279..511l&link_type=abstract

Monthly Notices of the Royal Astronomical Society, Volume 279, Issue 2, pp. 511-520.

Astronomy and Astrophysics

Astronomy

7

Atomic Processes, Planetary Nebulae: General, Planetary Nebulae: Individual: Ngc 6720, Planetary Nebulae: Individual: Ngc 7027

Scientific paper

Deep long-slit spectra have been obtained in order to map the electron temperature and density in the warm transition regions around the Ring Nebula using the optical [CI] and [NI] forbidden lines. For the first time the [CI] lambda8727 line is detected and mapped in this nebula. The temperature-sensitive [CI] nebular to auroral line ratio (lambda9824+lambda9850)/lambda8727 yields a mean electron temperature of 8250K, while variations of up to 2000K can be seen for emission from the different parts of the nebula. The electron density derived from the density-sensitive [NI] doublet ratio, lambda5198/lambda5200, is similar to that deduced for the fully ionized regions using the [ClIII] lambda5517/lambda5537 doublet ratio. As compared with lines from ionized regions, the [CI] and [NI] lines show dramatic and complex variations, both in their surface brightness distributions and in their radial velocities along the nebular minor axis, in a manner largely consistent with the bipolar model proposed by Bryce, Balick & Meaburn for the Ring Nebula. The bulk material movement revealed by the large outflow velocities in the transition regions (up to +/-35 km s^-1), relative to the ionized regions, is likely to generate strong shocks, and thus provides a natural excitation mechanism for the strong near-infrared H_2 emission lines observed in the Ring Nebula. We have also observed NGC 7027. The new observations confirm the earlier results of Danziger & Goad, who found a very small (lambda9824+lambda9850)/lambda8727 ratio for NGC 7027 and correctly attributed it as due to collisional de-excitation of the upper levels of the [CI] nebular lines under the very high-density conditions found in NGC 7027.

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