Stellar flares cannot explain the Galactic 511 keV emission

Context. The origin of the 511 keV line signal in the Milky Way remains unresolved despite decades of observations. The measured flux of ∼3 × 10 −3 ph cm −2 s −1 suggests a steady-state positron injection rate of ∼10 43 -10 44 e + s −1 . One proposed contributor to this signal is stellar flaring activity since high-energy solar flares are known to produce positrons and associated annihilation radiation. Aims. We estimated the quasi-persistent 511 keV luminosity expected from flaring stellar populations and estimated the Galactic contribution. We constrained the 511 keV fluxes for different sources in the Galaxy, in particular for globular clusters. Methods. Using solar flare observations as a calibration baseline, we constructed a hierarchical Bayesian model to link flare energy to 511 keV luminosity. We further used flare frequency-energy distributions to estimate the time-averaged positron output of stellar populations. We compared the resulting predictions with INTEGRAL/SPI observations using spatial and population-based models. Results. We find that stellar flares fall short by several orders of magnitude in explaining the Galactic positron annihilation rate. Reproducing ∼10% of the observed luminosity in the Galactic bulge would require unphysically large maximum flare energies per star reaching up to E max ≳ 10 37−39 erg. Spatial modeling further shows that stellar flare scenarios cannot reproduce the observed 511 keV morphology. Conclusions. Stellar flares cannot be the dominant source of Galactic positrons. Although previous studies have shown that the measured 511 keV morphology is broadly consistent with old stellar populations, our results exclude normal stellar flaring activity as the underlying source for this emission.