Hydrodynamical simulations of proton ingestion flashes in Type I X-ray Bursts

TL;DR

This paper presents multidimensional fluid simulations of helium ignition in neutron star atmospheres, revealing complex convection and mixing processes that differ from traditional models, with implications for understanding X-ray bursts.

Contribution

First multidimensional simulations of proton ingestion flashes in Type I X-ray bursts, highlighting new convection and mixing behaviors not captured by 1D models.

Findings

Convection zone grows outward due to cooling of fluid elements.

Efficient mixing transports carbon and protons before contact with hydrogen shell.

Differences from previous 1D simulations impact future modeling and observations.

Abstract

We perform the first multidimensional fluid simulations of thermonuclear helium ignition underneath a hydrogen-rich shell. This situation is relevant to Type I X-ray bursts on neutron stars that accrete from a hydrogen-rich companion. Using the low Mach number fluid code MAESTROeX, we investigate the growth of the convection zone due to nuclear burning, and the evolution of the chemical abundances in the atmosphere of the star. We also examine the convective boundary mixing processes that cause the evolution to differ significantly from previous one-dimensional simulations that rely on mixing-length theory. We find that the convection zone grows outwards as penetrating fluid elements cool the overlying radiative layer, rather than directly from the increasing entropy of the convection zone itself. Simultaneously, these flows efficiently mix composition, carrying carbon out of, and protons into the convection zone even before contact with the hydrogen shell. We discuss the implications of these effects for future modeling of these events and observations.