Core Helium Flash in Low-Mass Stars

• Core helium flash is a sudden ignition of helium burning in degenerate cores of low-mass red giants that transforms them into convective, stable structures.
• Multidimensional hydrodynamic simulations reveal turbulent entrainment and complex convection that drive observable asteroseismic signatures and chemical mixing.
• The flash triggers rapid mixing, lithium enrichment via the Cameron–Fowler process, and envelope expansion that can lead to binary interactions and transient phenomena.

A core helium flash is a thermonuclear runaway event that ignites helium burning in the highly degenerate core of a low-mass star at the tip of the red giant branch (RGB). This process marks a pivotal transition in stellar evolution, converting an electron-degenerate helium core into a convective, nondegenerate structure, and heralding the star’s entry onto the horizontal branch or red clump as a core helium-burning object. The multidimensional, highly dynamic nature of the helium flash, its progenitor dependence, and its varied observational consequences have been elucidated in detail by hydrodynamic simulations, asteroseismology, and nucleosynthetic constraints over the past decade.

1. Physical Conditions and Ignition Mechanism

In stars with mass $M_* \lesssim 2\,M_\odot$, the inert helium core grows in mass during the RGB phase and becomes supported almost exclusively by electron degeneracy pressure. Helium fusion via the $3\alpha$ process requires temperatures of $T \sim 10^8$ K, but under degenerate conditions, pressure is only weakly temperature-dependent. Once ignition occurs, there is no immediate expansion and cooling to moderate the burning; the result is a thermonuclear runaway, termed the core helium flash (Gautschy, 2012).

Neutrino cooling is highly effective at the center of the core, shifting the temperature maximum off-center (typically at a fractional mass coordinate $q \sim 0.17$ in a $1.3\,M_\odot$ model), establishing the location for runaway helium burning. The ignition region remains nearly at constant density during the flash, and the local path in the $(\log\rho,\,\log T)$ plane is near-vertical (adiabatic slope $\approx 2/3$). Once a threshold is reached, degeneracy is lifted in the ignition region, and convection develops rapidly, distributing energy throughout the core (Gautschy, 2012).

2. Multidimensional Hydrodynamics and Convection

One-dimensional (1D) stellar evolution codes traditionally model the flash with time-dependent algorithms that cannot capture the full range of turbulent, dynamical behavior. High-resolution two- and three-dimensional hydrodynamic simulations, using codes such as Herakles, have revealed that convection during the core helium flash is inherently multi-dimensional and characterized by vigorous turbulent entrainment at convective boundaries (Mocak et al., 2010, Mocak et al., 2011).

• In metal-rich (Pop I) stars, a single convection zone forms and can expand outward over time, potentially reaching the hydrogen-rich envelope within $\sim$23 days.
• In metal-free (Pop III) stars, the convection zone quickly bifurcates: there is an inner, helium-burning driven zone and an outer, hydrogen-burning driven zone (split by a radiative barrier), but this double zone is dynamically transient and soon decays into a regime dominated by internal gravity waves.

Convective velocities match or exceed those predicted by mixing-length theory, but nonradial instabilities, boundary overshoot, and entrainment yield a growing, non-barriered zone. Quantitatively, the entrainment velocity at the convective boundary is $u_e = \Delta q / (h N^2)$, where $\Delta q$ is the buoyancy flux jump, $h$ the boundary thickness, and $N$ the Brunt–Väisälä frequency (Mocak et al., 2010).

3. Turbulent Entrainment, Entropy Barriers, and Mixing

Multidimensional calculations demonstrate that entropy barriers assumed in 1D models (i.e., sharp entropy steps at the convective boundary) are permeable to turbulent entrainment. Even in the presence of a substantial entropy gradient ($\sim$20% at the H–He boundary in Pop I stars), convection steadily erodes the interface, and hydrogen can be mixed into the helium-burning convection zone—a process not efficiently captured by 1D overshoot or ad hoc mixing prescriptions (Mocak et al., 2011).

This “hydrogen injection flash” results in rapid nuclear energy release, the appearance of a secondary convection zone in the hydrogen envelope, and the development of dual or retrenching convective shell structures. This mixing process physically underpins changes in surface composition, contributes to peculiar abundance patterns, and initiates complex thermal and compositional feedbacks (Mocak et al., 2011, Mocak et al., 2011).

Additionally, simulations have revealed a new dynamic mixing process below the base of shell convection zones during the flash, involving cold, dense blobs enriched in higher mean molecular weight material sinking from the radiative region—a process aided by inverse $\nabla_\mu$ gradients and shear-induced instabilities (Mocak et al., 2011).

4. Seismic Diagnostic and Observational Signatures

The helium flash phase, which lasts $\sim$2 Myr (core helium flash + subflashes), leaves characteristic asteroseismic signatures. Following the flash, the contraction of the envelope increases the coupling between envelope acoustic ($p$-) and core gravity ($g$-) modes, making it possible to detect mixed $\ell=1$ modes in photometric surveys like Kepler and CoRoT (Bildsten et al., 2011).

Distinctive observables include:

• Gravity-mode period spacings $\Delta P_g$ evolving from $\sim$61 s (degenerate core) to $70$–$100$ s during the flash, finally reaching $\sim$255 s in the post-flash, nondegenerate red clump.
• Large frequency separation $\Delta\nu$ variations due to rapid envelope contraction and radius change.
• Short-lived, phase-specific oscillation patterns, such as additional inner-core g-mode signatures during a subflash, measurable rotational splittings for different g-mode cavities, and buoyancy glitches at structural boundaries (Deheuvels et al., 2018).

Quantitatively, the JWKB approximation yields mode-matching conditions involving coupled phase shifts and coupling coefficients, capturing the complex spectral structure of red giants during flash-driven convection (Deheuvels et al., 2018).

5. Mixing, Lithium Enrichment, and the Cameron–Fowler Process

The core helium flash can induce efficient mixing events, particularly during the most energetic first subflash, enabling internal gravity waves (IGWs) excited by core convection to transport 7Be from above the H-burning shell into the envelope, where it decays to 7Li (the Cameron–Fowler mechanism). This process naturally explains the observed lithium enhancement in red clump stars—A(Li) values increasing by a factor of $\sim$40 between the tip of the RGB and the red clump, as deduced by mixing models (Schwab, 2020).

• Effective mixing requires diffusion coefficients $$D_\mathrm{mix} \sim 10^{11}\,\mathrm{cm}^2\,\mathrm{s}^{-1}$$ and IGW-driven mixing sufficient to overcome local radiative or compositional barriers.
• Some simulations indicate that, for typical wave luminosities from HeCZ convection, the newly formed convection zone in the H-burning shell often forms too deep to merge with the surface envelope, potentially leading only to modest lithium enhancement (A(Li) $\lesssim$ 1.5), but sufficient wave-induced mixing or favorable evolutionary circumstances can lead to “super–Li-rich” giants ($A(\rm Li) > 3$) (2206.13479, Lu et al., 8 Apr 2025).

The lithium-rich phase after the flash is typically short-lived compared to the red clump lifetime ($\sim$2–15 Myr), a fact consistent with the rarity of Li-rich red clump stars (Lu et al., 8 Apr 2025).

6. Envelope Expansion, Binary Evolution, and Astrophysical Consequences

Wave energy generated by vigorous core convection during the helium flash is predicted to carry a substantial fraction ($L_\mathrm{wave} \sim 0.01 L_\mathrm{conv}$) of the flash’s total power outward to the envelope (Bear et al., 2021, Fainer et al., 2022). This energy deposition can:

• Inflate the envelope by tens to hundreds of solar radii over several years, driving transient luminosity increases, dust production, and mass loss (with $\Delta L$ up to a few $10^4$ $L_\odot$).
• Initiate common envelope evolution (CEE) in binaries, even in systems that would otherwise avoid it, thereby enabling planetary or brown dwarf engulfment and yielding exotic compact object companions (e.g., WD1856+534).
• Promote the formation of extreme horizontal branch (EHB) stars through enhanced envelope loss and modify planetary nebula morphologies via asymmetric mass loss (Fainer et al., 2022). The criterion for CEE initiation can be formalized as $a(1-e) < R_1$.

Such events may manifest as months- to years-long, faint, red infrared transients.

7. Neutrino Processes and Fundamental Physics Probes

Neutrinos play a dual role: thermal neutrinos (e.g., from plasmon decay) dominate pre-flash cooling, set core degeneracy, and thereby influence flash ignition mass and locus; nuclear neutrinos (from, for example, $^{18}$F decay during the nitrogen flash) contribute temporarily detectable fluxes during the main flash (Capelo et al., 2023).

• Core models including a nonzero neutrino magnetic moment (NMM) predict increased critical He core mass ($\sim$5% higher), elevated TRGB luminosity (by $\sim$35%), and an earlier, more vigorous, more off-center helium flash. Enhanced NMM also narrows the mixing bottleneck at the interface of radiative and convective zones, favoring rapid Li transport to the surface and the formation of super Li-rich red clump stars (Lu et al., 8 Apr 2025).

Asteroseismic precision at the level of $0.3\,\mu$Hz in $\Delta\nu$ is sufficient to distinguish between composition models and to identify stars in subflash phases; concurrent paper of neutrino fluxes and asteroseismic parameters thus constrains both stellar astrophysics and possible beyond-Standard-Model neutrino properties (Capelo et al., 2023).

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The contemporary understanding of the core helium flash is that of a dynamic, multidimensional event whose subsequent evolutionary and observational consequences—including envelope reconfiguration, mixing and nucleosynthesis, transient phenomena, and binary evolution—are centrally shaped by a complex interplay of hydrodynamics, wave excitation, neutrino physics, and stellar structure. This underscores the inadequacy of 1D models for capturing key processes and compels the use of multidimensional hydrodynamics, nucleosynthesis tracking, and asteroseismic analysis in theoretical and observational studies.