Dynamical Mass Loss at the End of TP-AGB stars

TL;DR

The paper investigates how TP-AGB stars lose their remaining envelopes at the end of their evolution by using one-dimensional hydrodynamic simulations in the MESA framework for a $1.5\,M_{\odot}$ star. It demonstrates that envelope mass strongly influences pulsation period and amplitude, with violent, envelope-ejecting pulsations occurring when the envelope mass falls below about $0.25\,M_{\odot}$, potentially removing the residual envelope in a few hundred years. This pulsation-driven mass loss offers a plausible mechanism for the rapid transition to the post-AGB phase, complementing dust-driven winds and dependent on initial mass and metallicity. The work highlights the need for a broader grid of models to fully map the parameter space and cautions about limitations from grey opacities and single-mass focus, while suggesting applicability to related stellar contexts such as red supergiants and common-envelope events.

Abstract

The thermally pulsating asymptotic giant branch (TP-AGB) phase plays a key role in the evolution of low- to intermediate-mass stars, driving mass loss that influences their final stages and contributes to galactic chemical enrichment. However, the mechanisms behind mass loss, particularly at the end of AGB, are still not well understood. We aim to investigate the relationship between stellar parameters and envelope dynamics during the TP-AGB phase, evaluating whether dynamical instabilities in the envelope can act as a possible mass-loss mechanism. We use hydrodynamics method in MESA to simulate the dynamical pulsations and resulting mass loss during the TP-AGB phase of a star evolved from a 1.5 Msun zero-age main sequence. Our simulations reproduce the dynamical pulsation behavior of stars during the TP-AGB phase, demonstrating that the envelope mass is a key factor governing pulsational properties. As the envelope mass decreases, both the pulsation period and radial amplitude increase, consistent with observational trends. For 1.5 Msun model, once the envelope mass declines to approximately 0.25 Msun, the model enters a regime of violent pulsations, potentially ejecting the remaining envelope within a few hundred years. We suggest that the instability can act as the dominant mass-loss mechanism in the end of the TP-AGB phase, marking a rapid transitional stage toward the post-AGB phase.

Figures (7)

• Figure 1: The hydrostatic evolution during the TP-AGB phase. The blue line represents the model's radius, while the red lines indicate the total mass and helium core mass, respectively. The green point mark the selected 0.64$\mathrm{M}_{\odot}$ envelope model for subsequent hydrodynamical simulations.
• Figure 2: The early stage of hydrodynamical simulations starting from a hydrostatic model with an envelope of 0.64$\mathrm{M}_{\odot}$. For simplicity, we set $t = 0$ at the beginning of the simulation. Panel (a) uses a time step of $10^{-3}$ years, while panel (b) uses a time step of $10^{-2}$ years.
• Figure 3: Hydrodynamical evolution of the model with an initial envelope mass of 0.64$\mathrm{M}_{\odot}$. The model first undergoes a relaxation phase to reach stable pulsations. At around $t \approx 120$ years, a constant mass-loss rate of $10^{-3}$$\mathrm{M}_{\odot}/\mathrm{yr}$ is introduced to reduce the envelope artificially.
• Figure 4: Hydrodynamical evolution of models with different initial envelope masses. Panels (a), (b), and (c) correspond to the initial envelope masses of 0.36, 0.25, and 0.2$\mathrm{M}_{\odot}$, respectively. The start of the simulation is set to $t = 0$ for all cases.
• Figure 5: Panels (a) and (b) show the evolution during the first four years after the onset of the dynamical simulation for models with masses of 0.64$\mathrm{M}_{\odot}$ and 0.25$\mathrm{M}_{\odot}$, respectively. The black solid line indicates the stellar radius, while the gray dashed lines mark the radial positions that divide the envelope mass into 20 equal parts. The colorbar represents the relative fraction of ionized hydrogen.