  For helium II inside a rotating cylinder, it is proposed that the formation
of vortex lines of the frictionless superfluid component of the liquid is
caused by the presence of the rotating quasi-particles gas. By minimising the
free energy of the system, the critical value Omega_0 of the angular velocity
for the formation of the first vortex line is determined. This value
nontrivially depends on the temperature, and numerical estimations of its
temperature behaviour are produced. It is shown that the latent heat for a
vortex formation and the associated discontinuous change in the angular
momentum of the quasi-particles gas determine the slope of Omega_0 (T) via some
kind of Clapeyron equation.
