Astronomy and Astrophysics – Astrophysics
Scientific paper
Feb 1979
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1979a%26a....72...31r&link_type=abstract
Astronomy and Astrophysics, vol. 72, no. 1-2, Feb. 1979, p. 31-38. Research supported by the Science Research Council.
Astronomy and Astrophysics
Astrophysics
25
Astronomical Models, Electronography, Planetary Nebulae, Astronomical Maps, Forbidden Bands, H I Regions, Helium, Line Spectra, Nitrogen, Optical Filters, Oxygen Spectra
Scientific paper
Electronographic observations of the low-excitation planetary nebula IC 418 through narrow-band interference filters are reported. The filters used correspond to the H-alpha line; the forbidden N II line at 6583 A; the 3868-A forbidden line of Ne III; the forbidden lines of O I, O II, and O III at 6300, 3726/3729, and 4959/5007 A, respectively; the 5876-A line of He I; the He II line at 4686 A; the forbidden S II line at 6731 A; and the continuum region centered at 4788 A. It is found that the structure of the nebula changes when viewed in lines of different excitation potential and that there is considerable overlap among the different stages of ionization of the various atoms. An oblate spheroidal model for the three-dimensional distribution of ionized gas within the nebula is derived which includes a region of lower density that extends inward toward the central star. The results suggest that unresolved filamentary structure is present within the envelope, that the low-density region is likely to be contiguous to the main shell and is probably taking part in the general expansion of the whole envelope, and that an H I region may be situated external to the nebula.
Reay N. K.
Worswick Susan P.
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