Astronomy and Astrophysics – Astronomy
Scientific paper
Sep 1979
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1979mnras.188..669f&link_type=abstract
Monthly Notices of the Royal Astronomical Society, vol. 188, Sept. 1979, p. 669-673.
Astronomy and Astrophysics
Astronomy
19
Astronomical Spectroscopy, Emission Spectra, Line Spectra, Planetary Nebulae, Electron Density Profiles, Electron Energy, Interstellar Gas, Radiation Distribution, Thermodynamic Equilibrium, Ultraviolet Radiation
Scientific paper
Recent observations of the compact planetary nebula IC 4997 have shown that the O III 4363/H-gamma forbidden-line intensity ratio can vary by as much as 20% within one year. In this note it is shown that this intensity ratio is insensitive to the gas density but that it is a sharp function of the electron temperature. The 4363/H-gamma ratio will change rapidly if the thermal equilibrium of the nebula is upset (say, by a change in the ionizing flux from the central star) because the cooling time of the gas is short. The rapid changes in the 4363/H-gamma ratio in IC 4997 are easily understood in terms of small (about 10%) changes in the number of ionizing photons and can provide a great deal of information concerning small changes in the ultraviolet radiation field of the central star. The need for further observations of this interesting object is stressed.
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