We present a new calculation of the neutrino flux received at Earth from a massive star in the \sim 24 hours 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 \mathrel { { \nu _ { e } } } flux has a high energy spectrum tail , at E \mathrel { \vbox { \hbox { $ > $ } \nointerlineskip \hbox { $ \sim$ } } } 3 - 4 MeV , which is mostly due to decay and electron capture on isotopes with A = 50 - 60 .
In a tentative window of observability of E \mathrel { \vbox { \hbox { $ > $ } \nointerlineskip \hbox { $ \sim$ } } } 0.5 MeV and t < 2 hours pre-collapse , the contribution of beta processes to the \mathrel { { \nu _ { e } } } flux is at the level of \sim 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 \sim 30 \% due to beta processes .
These processes dominate the signal at a liquid argon detector , thus greatly enhancing its sensitivity to a presupernova .