Detection horizon for the neutrino burst from the stellar helium flash

Abstract

Low-mass stars ($M\lesssim 2\,M_\odot$) ignite helium under degenerate conditions, eventually causing a nuclear run-away -- the helium flash. The alpha-capture process on $^{14}$N produces a large amount of $^{18}$F, whose subsequent decay spawns an intense $ν_e$ burst (with average energy of $0.38$ MeV) lasting about a day. We show that, in addition, a strong $1.7$ MeV neutrino line is generated by electron capture on $^{18}$F. Detection is hindered by large backgrounds in state-of-the-art neutrino observatories, such as JUNO. In next-generation facilities, such as the Jinping neutrino experiment, the horizon for a detection with a local significance of $3 σ$ would be extended to almost $3$ pc. Although helium flashes occur a few times per year in our Galaxy, there are no stellar candidates approaching the tip of the red giant branch within $10$ pc. Hence, to date, asteroseismology remains the most promising tool for probing the most energetic thermonuclear event in the life of a low-mass star.

Figures (7)

• Figure 1: Evolution of the rate of energy release during the He flash of three models with ZAMS mass of $1$, $1.8$, and $2 \,M_\odot$. The upper (thin) lines refer to the $3\alpha$ reaction, the lower (thick) lines to $^{14}{\rm N}(\alpha,\gamma)$, leading to ${}^{18}{\rm F}$ production. Time is relative to $t_{\rm peak}$, i.e., the instant of maximum $L_{3\alpha}$. The vertical lines show the time offset of the maximum of $L_{{\rm N}\alpha}$ relative to $t_{\rm peak}$.
• Figure 2: Core profiles of the $1\,M_\odot$ model for selected times before and after He ignition ($t-t_{\rm{peak}} = -3000$ years, $-5.5$ days, $-0.4$ days, and $7.8$ days). From top to bottom, we show the baryon density $\rho$, medium temperature $T$, the $^{14}$N abundance by mass, and the $3\alpha$ energy generation rate per unit mass. The inset of the bottom panel displays the energy generation rate in the proximity of $t_{\rm{peak}}$.
• Figure 3: Evolution of several physical quantities in the He-burning region, defined through Eq. \ref{['eq:Qav']}. From top to bottom, we show $\rho$, $T$, $X_{^{14}{\rm N}}$, the mass coordinate ($m$), the width of the relevant mass region ($\delta m$), and the radial coordinate $R$. The colored asterisks mark the snapshots shown in Fig. \ref{['fig:profiles']}, except for the pre-ignition profile at $-3000$ years.
• Figure 4: Evolution of $^{18}$F neutrino emission for our $1\,M_\odot$ model. Top panel: Production by the two channels and their sum. The vertical lines indicate $t_{\rm peak}$ (corresponding to the maximum of $L_{3\alpha}$) and the maximum of neutrino emission from EC. Bottom panel: Fractional contributions of $\beta^+$ decay and EC. At early times, EC dominates the neutrino emission rate; after the He flash, $\beta^+$ decay emission becomes the dominant process.
• Figure 5: Evolution of $^{18}$F neutrino emission from EC for our model with mass of $1\,M_\odot$. Top panel: Emission rate, identical to the orange line of Fig. \ref{['fig:emissionbetaec']}, now on a linear scale and centered on its maximum. Middle panel: Cumulative distribution relative to $t_{\nu\ \rm{EC\ peak}}$. Bottom panel: Cumulative rates divided by $\sqrt{|t|}$, with the maxima indicated by vertical lines. The maxima of $N_{1,2}/\sqrt{t_{1,2}}$ arise at $\tilde{t}_1=-0.96$ days and $\tilde{t}_2=2.15$ days, marked by vertical lines. The optimal window for neutrino searches is given by $\tau=\tilde{t}_2-\tilde{t}_1=3.11$ days.