Detection horizon for the neutrino burst from the stellar helium flash

Abstract

Low-mass stars ($M\lesssim 2M_{\odot }$) 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 ${\nu }_e$ 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 $3\sigma$ 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.

14 pages, 7 figures, 1 table, 2 appendices. Matches version accepted for publication in Phys.Rev.D

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