Multidimensional hydrodynamic simulations of the hydrogen injection flash

TL;DR

This study uses multidimensional hydrodynamic simulations to demonstrate that hydrogen can penetrate the helium-burning shell in stars, leading to a hydrogen injection flash, challenging previous one-dimensional models.

Contribution

It provides new evidence that multidimensional simulations reveal hydrogen mixing into the helium core, which is difficult to replicate in traditional 1D stellar models.

Findings

Hydrogen injection occurs across the entropy barrier in multidimensional simulations.

Multidimensional flow allows continuous hydrogen mixing into the helium core.

1D models struggle to reproduce the hydrogen injection flash observed in simulations.

Abstract

The injection of hydrogen into the convection shell powered by helium burning during the core helium flash is commonly encountered during the evolution of metal-free and extremely metal-poor low-mass stars. With specifically designed multidimensional hydrodynamic simulations, we aim to prove that an entropy barrier is no obstacle for the growth of the helium-burning shell convection zone in the helium core of a metal-rich Pop I star, i.e. convection can penetrate into the hydrogen-rich layers for these stars, too. We further study whether this is also possible in one-dimensional stellar evolutionary calculations. Our hydrodynamical simulations show that the helium-burning shell convection zone in the helium core moves across the entropy barrier and reaches the hydrogen-rich layers. This leads to mixing of protons into the hotter layers of the core and to a rapid increase of the nuclear energy production at the upper edge of the helium-burning convection shell - the hydrogen injection flash. As a result a second convection zone appears in the hydrogen-rich layers. Contrary to 1D models, the entropy barrier separating the two convective shells from each other is largely permeable to chemical transport when allowing for multidimensional flow, and consequently, hydrogen is continuously mixed deep into the helium core. We find it difficult to achieve such a behavior in one-dimensional stellar evolutionary calculations.