[Paper Review] Three Dimensional Simulations of the Core Helium Flash - with Rotation

John C. Lattanzio, David S. P. Dearborn+2 more|ArXiv.org|Dec 6, 2006

Solar and Space Plasma Dynamics3 citations

TL;DR

This study uses 3D hydrodynamical simulations with the Djehuty code to investigate the impact of rotation on the core helium flash in low-mass stars. Despite simulating rotation periods matching observed white dwarf rates (0.87–4.8 hours), the results show negligible structural changes, no enhanced mixing, and only minor luminosity variations (≤1%), indicating rotation has minimal influence during the early flash phase on the timescales studied.

ABSTRACT

We continue our study of the core helium flash using the three dimensional hydrodynamics code Djehuty. Continuing from earlier calculations, we now take relaxed 3D configurations and add various amounts of rotation. We find that rotation periods consistent with those observed in white dwarfs produce negligible changes in the structure and evolution of the core flash, at least for the very small timescales we have yet been able to investigate. There is no sign of any extra mixing due to the rotation. There is some inconclusive evidence for a slight change in the luminosity, at the 1% level.

Motivation & Objective

• To investigate how rotation influences the dynamics and evolution of the core helium flash in low-mass stars.
• To assess whether rotation induces enhanced mixing or alters convective structure during the flash phase.
• To determine if observed white dwarf rotation periods produce measurable changes in the flash's energy output or convective extent.
• To evaluate the validity of 1D models under conditions of aspherical convection and rapid energy release.
• To explore the role of Coriolis forces in modifying convective plume dynamics and energy transport.

Proposed method

• Simulations employ the 3D ALE (Arbitrary Lagrange-Eulerian) hydrodynamics code Djehuty with second-order spatial and temporal accuracy.
• Initial 3D models are derived from relaxed 1D models using the same physics, minimizing initial transients from code transition.
• Solid-body rotation is imposed at three periods (4.8, 1.6, and 0.87 hours), consistent with observed white dwarf rotation rates.
• The code includes self-gravity (spherically symmetric), detailed EoS, conductive and radiative opacities, and a 21-species nuclear reaction network.
• Convection is initialized via buoyancy-driven flows, not predefined zones, to avoid artificial symmetry.
• Mixing is traced using 18O as a tracer, formed via α-capture on 14N, which is absent in the initial 1D model.

Experimental results

Research questions

• RQ1 Does rotation at observed white dwarf periods significantly alter the structure or convective extent of the core helium flash?
• RQ2 Is there evidence of enhanced mixing due to rotational effects during the flash phase?
• RQ3 How does rotation influence the luminosity evolution during the core helium flash?
• RQ4 To what extent do Coriolis forces modify convective plume trajectories and energy transport efficiency?
• RQ5 Do rotational effects lead to deviations from spherical symmetry in the flash dynamics?

Key findings

• Rotation periods consistent with observed white dwarfs (0.87–4.8 hours) produce negligible structural changes in the core helium flash, even over 2000 seconds of simulation.
• No evidence of enhanced mixing is observed; the extent of convection and 18O tracer distribution remain similar to the non-rotating case.
• Luminosity increases by up to 0.50% in the slowest-rotating case (4.8-hour period), but fluctuations are typically 0.36%, suggesting no robust trend.
• The 18O tracer reveals convective plumes rising ~3000 km in ~1 minute, with some twisting due to rotation, but no significant disruption or enhanced spreading.
• Convection extends inward by 458 km in 2000 seconds in the rotating case (Spin1), compared to 451 km in the non-rotating case, indicating no significant change in convective growth rate.
• Centrifugal forces are negligible (≤1% of gravity) even at the core edge, and rotational velocities (up to 12 km/s) are much smaller than convective speeds (~40 km/s).