  We explore the impact of small-scale flavor conversions of neutrinos, the
so-called fast flavor conversions (FFCs), on the dynamical evolution and
neutrino emission of core-collapse supernovae (CCSNe). In order to do that, we
implement FFCs in the spherically symmetric (1D) CCSN simulations of a 20
solar-mass progenitor model parametrically, assuming that FFCs happen at
densities lower than a systematically varied threshold value and lead to an
immediate flavor equilibrium consistent with lepton number conservation. We
find that besides hardening the electron neutrino and antineutrino spectra,
which helps the expansion of the shock by enhanced postshock heating, FFCs can
cause significant, nontrivial modifications of the energy transport in the SN
environment via increasing the heavy-lepton neutrino luminosities. In our
non-exploding models this results in extra cooling of the layers around the
neutrinospheres, which triggers a faster contraction of the proto-neutron star
and hence, in our 1D models, hampers the CCSN explosion. Although our study is
limited by the 1D nature of our simulations, it provides valuable insights into
how neutrino flavor conversions in the deepest CCSN regions can impact the
neutrino release and the corresponding response of the stellar medium.
