Multidimensional Modeling of Type I X-ray Bursts. II. Two-Dimensional Convection in a Mixed H/He Accretor

TL;DR

This study uses two-dimensional simulations to investigate convection in mixed hydrogen/helium layers on neutron stars during Type I X-ray bursts, revealing a layered convective structure influenced by nuclear processes.

Contribution

It introduces a 2D modeling approach with the Maestro code to simulate low Mach number convection in XRBs, highlighting the effects of nuclear reaction networks on convective layering.

Findings

Convective region splits into two layers due to nuclear processes.

Approximate rp-process network influences convective structure.

Preliminary results suggest need for improved nuclear reaction modeling.

Abstract

Type I X-ray Bursts (XRBs) are thermonuclear explosions of accreted material on the surfaces of a neutron stars in low mass X-ray binaries. Prior to the ignition of a subsonic burning front, runaway burning at the base of the accreted layer drives convection that mixes fuel and heavy-element ashes. In this second paper in a series, we explore the behavior of this low Mach number convection in mixed hydrogen/helium layers on the surface of a neutron star using two-dimensional simulations with the Maestro code. Maestro takes advantage of the highly subsonic flow field by filtering dynamically unimportant sound waves while retaining local compressibility effects, such as those due to stratification and energy release from nuclear reactions. In these preliminary calculations, we find that the rp-process approximate network creates a convective region that is split into two layers. While this splitting appears artificial due to the approximations of the network regarding nuclear flow out of the breakout reaction 18Ne(a,p)21Na, these calculations hint at further simplifications and improvements of the burning treatment for use in subsequent calculations in three dimensions for a future paper.