Neutrinos from Beta Processes in a Presupernova: Probing the Isotopic Evolution of a Massive Star
We present a new calculation of the neutrino flux received at Earth from a massive star in the ~24 hr of evolution prior to its explosion as a supernova (presupernova). Using the stellar evolution code MESA, the neutrino emissivity in each flavor is calculated at many radial zones and time steps. In addition to thermal processes, neutrino production via beta processes is modeled in detail, using a network of 204 isotopes. We find that the total produced ${\nu }_{e}$ flux has a high-energy spectrum tail, at $E\gtrsim 3\mbox{--}4\,\mathrm{MeV}$, which is mostly due to decay and electron capture on isotopes with $A=50\mbox{--}60$. In a tentative window of observability of $E\gtrsim 0.5\,\mathrm{MeV}$ and $t\lt 2$ hr pre-collapse, the contribution of beta processes to the ${\nu }_{e}$ flux is at the level of ~90%. For a star at D = 1 kpc distance, a 17 kt liquid scintillator detector would typically observe several tens of events from a presupernova, of which up to ~30% is due to beta processes. These processes dominate the signal at a liquid argon detector, thus greatly enhancing its sensitivity to a presupernova.