Ejection of the Envelope

The greatly increased surface mass loss that characterizes AGB stars is not well understood. Instabilities associated with thermal pulses in the red giant may play a role, but the details are murky. Irrespective of the detailed mechanism, it is clear that the mass ejection rate in the AGB phase is so high that the star loses much of its envelope in a period of order 10,000 years. This is an incredibly short time compared with the lifetime of the star, which is roughly a million times longer. If we imagine the entire lifetime of such a star to be compressed into a single year, in the final seconds of December in our imaginary year the AGB star would be shedding its envelope at a rate of two solar masses per second!
Implications of Mass Loss
The mass loss from the surface has at least three important consequences:

  • The rapid loss of the envelope leaves behind a carbon-oxygen core that will become a white dwarf.
  • The ejected envelope is a likely cause of planetary nebulae.
  • The ejected material is a major source of the gas and dust grains found in the interstellar medium, particularly for compounds of carbon and related elements.

    • An AGB star's envelope is consumed from the inside by shell sources burning outward and from the outside by strong stellar winds. However, calculations show that the loss of the envelope to the advancing shell sources is negligible compared with the loss due to winds.

    Ionization of the Envelope
    In the late AGB mass ejection phase, the rate can increase even more in what is called a superwind. This superwind can overtake the previously ejected material, forming a shock wave that defines the inner boundary of the receding envelope. Once much of the insulating blanket of the envelope is stripped off, the hot carbon-oxygen core that is left behind is exposed and the spectrum of the central star shifts far into the ultraviolet. This bath of UV photons excites the atoms of the receding envelope (primarily hydrogen), and the light emitted when electronic transitions de-excite these atoms is responsible for the beautiful colors of planetary nebulae. The receding nebula will gradually disperse, enriching the interstellar medium in the processed material of the ejected envelope.