Helium shell flashes and ionization of planetary nebulae.
Abstract
It is noted that nuclei of planetary nebulae are expected to go through thermal pulses due to helium shell flashes. The time scale of a flash is comparable with the recombination time in a typical planetary nebulae and the nebular gas can then be far from equilibrium conditions. Attention is given to theoretical models constructed to study evolution of ionization and thermal structure of nebulae in which the ionizing radiation varies with time. It is concluded that several observational characteristics of planetary nebula, such as the occurrence of nebulae around cool stars, double-shell structures in planetaries, and coexistence of low and high excitation features in spectra of some nebulae can be explained by time dependent effects induced in nebulae by helium shell flashes in their nuclei.
- Publication:
-
Acta Astronomica
- Pub Date:
- 1979
- Bibcode:
- 1979AcA....29..355T
- Keywords:
-
- Astronomical Models;
- Flare Stars;
- Gas Ionization;
- Helium;
- Planetary Nebulae;
- Stellar Envelopes;
- Atomic Excitations;
- Cool Stars;
- Flash;
- Gas Temperature;
- Photoionization;
- Stellar Luminosity;
- Thermodynamic Equilibrium;
- Time Dependence;
- Astrophysics