3D hydrodynamical simulations of a proton ingestion episode in a low-metallicity asymptotic giant branch star

TL;DR

This study uses 3D hydrodynamical simulations to model proton ingestion in a low-metallicity AGB star, revealing complex convective behaviors and higher hydrogen burning than 1D models, challenging traditional mixing assumptions.

Contribution

First 3D simulations of proton ingestion in low-metallicity AGB stars showing detailed convective flows and higher hydrogen burning luminosities than 1D models.

Findings

Fast downward plumes transport hydrogen near helium burning shell.

3D models show higher hydrogen-burning luminosities than 1D models.

Convective velocities are over 10 times greater than mixing length theory predictions.

Abstract

We use the 3D stellar structure code DJEHUTY to model the ingestion of protons into the intershell convection zone of a 1 solar mass asymptotic giant branch star of metallicity Z=10^-4. We have run two simulations: a low resolution one of around 300,000 zones, and a high resolution one consisting of 2,000,000 zones. Both simulations have been evolved for about 4 hours of stellar time. We observe the existence of fast, downward flowing plumes that are able to transport hydrogen into close proximity to the helium burning shell before burning takes place. The intershell in the 3D model is richer in protons than the 1D model by several orders of magnitude and so we obtain substantially higher hydrogen-burning luminosities -- over 10^8 solar luminosities in the high resolution simulation -- than are found in the 1D model. Convective velocities in these simulations are over 10 times greater than the predictions of mixing length theory, though the 3D simulations have greater energy generation due to the enhanced hydrogen burning. We find no evidence of the convective zone splitting into two, though this could be as a result of insufficient spatial resolution or because the models have not been evolved for long enough. We suggest that the 1D mixing length theory and particularly the use of a diffusion algorithm for mixing do not give an accurate picture of these events. An advective mixing scheme may give a better representation of the transport processes seen in the 3D models.