Electron fraction constraints based on nuclear statistical equilibrium with beta equilibrium

The electron-to-nucleon ratio or electron fraction is a key parameter in many astrophysical studies. Its value is determined by weakinteraction rates that are based on theoretical calculations subject to several nuclear physics uncertainties. Consequently, it is important to have a model independent way of constraining the electron fraction value in different astrophysical environments. Here we show that nuclear statistical equilibrium combined with beta equilibrium can provide such a constraint. We test the validity of this approximation in presupernova models and give lower limits for the electron fraction in type Ia supernova and accretion-induced collapse.