Defining the Termination of the Asymptotic Giant Branch

TL;DR

This paper proposes a theoretical, quantitative criterion based on the ratio of dynamical to thermal timescales to define the end of the asymptotic giant branch (AGB) phase and the start of the post-AGB phase.

Contribution

It introduces a new criterion using the maximum of the ratio Q to determine the AGB termination, linking it to stellar temperature and contraction.

Findings

Q reaches its maximum at T~4000-6000 K.

Q correlates with stellar effective temperature and mass loss.

The criterion effectively captures the transition phase.

Abstract

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 time scale to the the envelope thermal time scale, Q, reaches its maximum value. Time average values are used for the different quantities, as the criterion does not refer to the short time-scale 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 K, it is related to the mass loss properties, and it reaches its maximum value when rapid contraction starts and envelope mass is very low.