On the rapid emission-line variation in IC 4997.
Abstract
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.
- Publication:
-
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- September 1979
- DOI:
- 10.1093/mnras/188.3.669
- Bibcode:
- 1979MNRAS.188..669F
- Keywords:
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- Astronomical Spectroscopy;
- Emission Spectra;
- Line Spectra;
- Planetary Nebulae;
- Electron Density Profiles;
- Electron Energy;
- Interstellar Gas;
- Radiation Distribution;
- Thermodynamic Equilibrium;
- Ultraviolet Radiation;
- Astrophysics;
- Electron Temperatures:Planetary Nebulae;
- Emission Lines:Planetary Nebulae;
- Planetary Nebulae:Variations