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

Liu, XW; Barlow, MJ; (1996) Physical conditions in the transition regions around the Ring Nebula and NGC 7027. Monthly Notices of the Royal Astronomical Society , 279 (2) pp. 511-520. 10.1093/mnras/279.2.511. Green open access

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Abstract

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 [C I] and [N I] forbidden lines. For the first time the [C I] λ8727 line is detected and mapped in this nebula. The temperature-sensitive [C I] nebular to auroral line ratio (λ9824 + λ9850)/λ8727 yields a mean electron temperature of 8250 K, while variations of up to 2000 K can be seen for emission from the different parts of the nebula. The electron density derived from the density-sensitive [N I] doublet ratio, λ5198/λ5200, is similar to that deduced for the fully ionized regions using the [Cl III] λ5517/λ5537 doublet ratio. As compared with lines from ionized regions, the [C I] and [N I] 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 H2 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 (λ9824 + λ9850)/λ8727 ratio for NGC 7027 and correctly attributed it as due to collisional de-excitation of the upper levels of the [C I] nebular lines under the very high-density conditions found in NGC 7027.

Type: Article
Title: Physical conditions in the transition regions around the Ring Nebula and NGC 7027
Open access status: An open access version is available from UCL Discovery
DOI: 10.1093/mnras/279.2.511
Publisher version: https://doi.org/10.1093/mnras/279.2.511
Language: English
Additional information: This version is the version of record. For information on re-use, please refer to the publisher’s terms and conditions.
UCL classification: UCL
UCL > Provost and Vice Provost Offices
UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences > Dept of Physics and Astronomy
URI: https://discovery.ucl.ac.uk/id/eprint/10096140
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