I suggest a theoretical quantitative definition for the termination of the asymptotic giant branch (AGB) phase and the beginning of the post-AGB phase. I suggest that the transition will be taken to occur when the ratio of the dynamical timescale to the envelope thermal timescale, Q = τdyn/ τkh-ev, reaches its maximum value. Time average values are used for the different quantities, as the criterion does not refer to the short-timescale variations occurring on the AGB and post-AGB, e.g., thermal pulses (helium shell flashes) and magnetic activity. Along the entire AGB the value of Q increases, even when the star starts to contract. Only when a rapid contraction starts does the value of Q start to decrease. This criterion captures the essence of the transition from the AGB to the post-AGB phase, because Q is connected to the stellar effective temperature, reaching its maximum value at T ≃ 4000-6000, it is related to the mass loss properties, and it reaches its maximum value when rapid contraction starts and envelope mass is very low.
- Stars: AGB and post