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The planetary nebula NGC 3918
Clegg, R. E. S.; Harrington, J. P.; Barlow, M. J.; Walsh, J. R.
AA(University College, London, England), AB(University College, London, England), AC(Maryland, University, College Park), AD(Anglo-Australian Observatory, Epping, Australia)
Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 314, March 15, 1987, p. 551-571. SERC-supported research. (ApJ Homepage)
Publication Date:
NASA/STI Keywords:
Astronomical Spectroscopy, Planetary Nebulae, Ultraviolet Astronomy, Abundance, Astronomical Maps, Flux Quantization, Iue, Line Spectra, Oxygen Spectra, Positive Ions, Southern Sky
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A detailed study of the planetary nebula NGC 3918, based on UV, optical, and radio observations, is presented. The central star has a predicted V magnitude of 14.6 and a luminosity of 6000 solar luminosities for a distance of 1.5 kpc. A contour map of the nebula in H-beta light is given. Velocity profiles of forbidden O II optical lines show that O(+) ions are mostly located at the front and rear of the nebula. A composite, biconical model is constructed which consists of optically thick cones at the front and rear with a low-density 'equatorial' region. The nebular expansion age of about 3000 yr is in reasonable agreement with the age of the nucleus found from evolutionary tracks. Silicon, magnesium, and iron are depleted by factors of 4, 3, and 100 respectively. It is shown that the Mg II 2800 A lines are affected by interstellar absorption in this and many other planetaries and thus often cannot be used for abundance determinations.

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