The Impact of Neutrino Magnetic Moments on the Evolution of the Helium Flash and Lithium-Rich Red Clump Stars

TL;DR

This work investigates how a non-zero neutrino magnetic moment ($\mu_v$) modifies low-mass stellar evolution, focusing on the helium flash, TRGB luminosity, and lithium enrichment in red clump stars. It implements $NMM$-driven energy losses (plasmon decay and pair processes) and an IGW-based mixing prescription within the MESA framework, exploring a range of $μ_{12}$ values and metallicities. The findings show that larger $μ_{12}$ increases the required helium-core mass for ignition and boosts TRGB luminosity (up to ~35% for typical 1 M$_\odot$ stars) while causing the helium flash to occur earlier with higher peak luminosities and more off-center, multi-flash behavior; IGW mixing can then transport Li to the surface, with the combination of $NMM$ and IGW effectively producing super Li-rich RCs. These results imply that precise asteroseismic measurements and Li abundances in RC stars could constrain the neutrino magnetic moment, offering a stellar evolution pathway to probe beyond-Standard-Model physics.

Abstract

The detection of the neutrino magnetic moment (NMM,$μ_v$) is one of the most significant challenges in physics. The additional energy loss due to NMM can significantly influence the He flash evolution in low-mass stars. Using the MESA code, we investigated the impact of NMM on the He flash evolution in low-mass stars. We found that NMM leads to an increase in both the critical He core mass required for the He flash and the luminosity of TRGB. For a typical $Z = 1 Z_{\odot}$ , $M$ = 1.0 $M_{\odot}$, and $μ_v = 3 \times 10^{-12} μ_{\mathrm{B}}$ model, the He core mass increases by $\sim 5\%$, and the TRGB luminosity increases by $\sim 35\%$ compared to the model without NMM. However, contrary to previous conclusions, our model indicates that the He flash occurs earlier, rather than delayed, with increasing NMM values. This is because the additional energy loss from NMM accelerates the contraction of the He core, releases more gravitational energy that heats the H shell and increases the hydrogen burning rate, thereby causing the He core to reach the critical mass faster and advancing the He flash. An increase in NMM results in a higher peak luminosity for the first He flash, a more off-center ignition position, and sub-flashes with higher luminosities, shorter intervals, and higher frequency. We found that the internal gravity wave (IGW) mixing generated by the He flash can induce sufficient mixing in the radiative zone, turning the overshoot region into a low-Dmix bottleneck within the stellar interior. The increase in NMM in the model narrows the overshoot bottleneck region, enabling Li to enter the surface convection zone more quickly, thereby enhancing the enrichment effect of IGW mixing on surface Li. For models incorporating both NMM and IGW mixing, the reduction in the overshoot bottleneck region allows them to effectively produce super Li-rich red clump star samples.

Figures (6)

• Figure 1: The structures and evolutions of models with different NMM values at $Z = 1 Z_{\odot}$, 0.01 $Z_{\odot}$ and $M$ = 1.0, 1.4, 1.8 $M_{\odot}$ before the helium flash. The left figure shows the models with $Z = 1\,\ Z_{\odot}$, while the right figure shows the model with $Z = 0.01\,\ Z_{\odot}$. The x-axis in all panels represents the age in $10^8$ years. In the top panel, the y-axis shows the helium core mass. In the middle panel, the y-axis depicts both the central temperature and the core boundary temperature, presented on a logarithmic scale. The y-axis in the bottom panel represents the central density, also shown on a logarithmic scale.
• Figure 2: The structures and evolutions of models with different NMM values at $Z = 1 Z_{\odot}$and $M$ = 1.4 $M_{\odot}$ during the helium flash phase. The NMM values for the models in the top, middle, and bottom panels are set to ${\mu_{12}}=0.0$, ${\mu_{12}}=1.0$, ${\mu_{12}}=3.0$, respectively. $t_{He}$ represents the time of the helium flash occurs. The green shaded area indicates the convection zone, the orange shaded area shows the neutrino production rate in logarithmic coordinates, the dotted line marks the helium core boundary, and the blue and purple lines represent the helium luminosity and stellar radius, respectively, each corresponding to their own separate y-axes on the right.
• Figure 3: Same as Fig. \ref{['fig:2']}, but for the models with $Z = 0.01 Z_{\odot}$.
• Figure 4: Hertzsprung-Russell (HR) diagram and time-luminosity diagram for $Z = 1 Z_{\odot}$ stars with different masses and NMM values. $t_{He}$ represents the time of the helium flash occurs. Black, red, and blue lines represent models with $M = 1.0, 1.4, 1.8$$M_{\odot}$, respectively. Solid, dashed, and dotted lines represent models with ${\mu_{12}}=0.0, 1.0, 3.0$, respectively.
• Figure 5: the structural evolution of $Z = 1 Z_{\odot}$, $M = 1.0 M_{\odot}$ star models with different NMM values and with and without IGW mixing during the first helium flash, along with the internal structure at 50 years after the helium flash. In the left panel, the blue line represents the hydrogen-burning zone at the helium core boundary, and the vertical gray line marks the 50-year time point selected for the right panel. The green shaded area represents the convection zone, the orange shaded area in the left panel indicates the mixing coefficient, and the pink shaded area in the right panel indicates the Li-burning zone.