  We calculate the thermochemical equilibrium of the diffuse interstellar
medium, including ionization by a photon flux F_{nu} from neutrino decay. The
main heating mechanism considered is photoelectrons from grains and PAHs. For
the studied range of F_{nu} values, there always exists two regions of
stability (a warm and a cold phase) that can coexist in equilibrium if the
thermal interstellar pressure is between a maximum value P_{max} and a minimum
value P_{min}. High F_{nu} values (~10^4-10^5 cm^{-2} s^{-1}) can be consistent
with observed interstellar pressures only if more efficient sources are heating
the gas. It is shown that a neutrino flux increase (due, for example, to an
increase in the supernova explosion rate) may stimulate the condensation of
cold gas by decreasing P_{max} below the interstellar pressure value.

Astrophysics