The Crimson Kiss of Two Giants: Helium Detonation and High-Energy Neutrino Production

Abstract

The coalescence of degenerate helium cores during red giant collisions - a process we term erythrohenosis - introduces a novel class of transient astrophysical sources of high-energy neutrinos. Using stellar models generated with MESA and SPH simulations of the final inspiral phase, we develop a semi-analytical model to estimate the amount of hydrogen mixed into the cores, the energy release ($\approx 4.28 \times 10^{49}$ erg) that heats the remnant to $T_f \approx 5.3 \times 10^8$ K, the magnetic field amplification ($B \approx 1.77 \times 10^{10}$ G), and the resulting neutrino flux. We find that the predicted TeV--PeV neutrino signal can account for the diffuse neutrino flux observed by IceCube and demonstrate that a single merger event within $\sim 2$ Mpc would be detectable in this energy regime. Furthermore, we discuss the probability of a magnetized helium flash and assess the subsequent activation of the CNO cycle in the remnant core due to hydrogen mixing. In particular, neutrinos from the decay of $^{18}$F offer a direct observational test of the detonation. The simultaneous emission of high-energy hadronic neutrinos, gravitational waves, and -- if the optical depth permits -- an electromagnetic signal would constitute a unique multimessenger signature of red giant core collisions, positioning erythrohenosis events as exotic yet potentially observable phenomena in dense stellar systems.

Figures (8)

• Figure 1: The upper curves represent pressure and density as a function of enclosed mass, illustrating that the degenerate nucleus contains nearly the entire mass of the star. The lower curves represent the same quantities as a function of stellar radius; these remain constant within the nucleus, as expected for a red giant of this age.
• Figure 2: The solid red line represents temperature as a function of stellar radius, while the dashed lines show temperature as a function of enclosed mass. The temperature remains constant throughout the degenerate nucleus, reaching its maximum at the core boundary. An nearly isothermal region is visible between $2$--$3 \times 10^{-2}\text{ }R_\odot$, corresponding to the hydrogen-burning shell AmaroSeoane_2023. The upper curves display the Fermi temperature as a function of radius (red dots) and enclosed mass (dash-dotted lines). These quantities converge at $\sim 2\times10^{-2}\text{ }R_\odot$, approximately the size of the stellar nucleus.
• Figure 3: The upper panel displays the hydrogen and helium mass fractions as a function of stellar radius, clearly delineating the degenerate helium core; hydrogen becomes the predominant element upon reaching the burning shell. The lower panel shows the abundances of $^{3}\mathrm{He}$, $^{12}\mathrm{C}$, $^{14}\mathrm{N}$, $^{16}\mathrm{O}$, $^{20}\mathrm{Ne}$, and $^{24}\mathrm{Mg}$ from the MESA models. While the abundances of the latter two isotopes remain constant throughout the star, the others exhibit transitions at the hydrogen-burning shell, marking the boundary of the helium-degenerate region.
• Figure 4: The relative velocity between the degenerate helium cores prior to merger. A non-zero eccentricity is evident; for this study, we adopt a characteristic value of $v_{\rm rel} = 100$ km s$^{-1}$. This velocity determines the kinetic energy $E_{\rm coll}$ used in subsequent calculations.
• Figure 5: Evolution of the magnetic field for several values of the growth parameter $\lambda_{\alpha}$. For values $\leq 0.5$, saturation is not reached efficiently within the characteristic $\sim 100$ s timescale of the phenomenon.