Universality of the neutrino collisional flavor instability in core-collapse supernovae

Neutrinos are known to undergo flavor conversion processes among the three flavors. The fast flavor conversion (FFC) has been the central piece of flavor conversions taking place in core-collapse supernovae (CCSNe) due to its shorter timescale to the completion of flavor conversion compared to other types of flavor conversion. Although the ordinary collisions between neutrinos and matter were once thought to decohere neutrinos and thus damp flavor conversions, it was recently realized that they can also induce the flavor conversion. The linear analysis showed that the so-called collisional flavor instability or CFI occurs in the absence of FFC. In this paper, we investigate if CFI takes place in of the postbounce core of CCSNe, using the results of spherically symmetric Boltzmann simulations of CCSNe for four progenitor models with different masses. We also provide an empirical correlation between matter properties and the occurrence of CFI in optically thick and semitransparent regions; baryon mass density (ρ ), electron fraction (Y_e), and the degeneracy of electron-type neutrinos (η_νe)need to be 10¹⁰ g^{/cm 3} ≲ρ ≲10¹² g^{/cm 3} , Y_e≲0.4 , and η_{ν e}≲0.5 , respectively. This condition allows us to easily locate the place of possible CFI occurence without detailed stability analyses, which is useful for analyzing CFI in CCSN models phenomenologically.