Formation of Planetary Nebulae.

It is suggested that planetary nebulae originate from extreme red giants that have become dynamically unstable. This instability arises when the mass contained in the hydrogen ionization zone is sufficient to reduce the effective ratio of specific heats for the envelope below ~. An approximate calculation shows that for a star with mass M= 1Mo, with a radiative core of mass=0.3 M, and with X =0.7 in the envelope, the instability first occurs at a radius R~10~ Ro. If we make the crude assumptions of adiabatic changes and homologous displacements, we may follow the growth of the instability using the virial theorem. With these assumptions, we find for the above example that the envelope is ejected from the star and that its final kinetic energy of expansion is~2 X 1048 erg. This energy and that required to overcome the gravitational binding of the envelope come from the energy released as ionized hydrogen and helium atoms in the envelope recombine. In this example, the predicted kinetic energy of expansion corresponds to a velocity 17 km/sec, which agrees well with the observed expansion velocities of planetary nebulae. We also note that this suggestion seems to meet the conditions inferred about the origin of planetary nebulae by Abell and Goldreich (Pubi. Astron. Soc. Pac~flc 78, 232,1966). This work has been partially supported by the Xational Science Foundation under Grant GP-5568.