  The formation mechanism of the jet-aligned CO clouds found by NANTEN CO
observations is studied by magnetohydrodynamical (MHD) simulations taking into
account the cooling of the interstellar medium. Motivated by the association of
the CO clouds with the enhancement of HI gas density, we carried out MHD
simulations of the propagation of a supersonic jet injected into the dense HI
gas. We found that the HI gas compressed by the bow shock ahead of the jet is
cooled down by growth of the cooling instability triggered by the density
enhancement. As a result, cold dense sheath is formed around the interface
between the jet and the HI gas. The radial speed of the cold, dense gas in the
sheath is a few km/s almost independent of the jet speed. Molecular clouds can
be formed in this region. Since the dense sheath wrapping the jet reflects
waves generated in the cocoon, the jet is strongly perturbed by the vortices of
the warm gas in the cocoon, which breaks up the jet and forms a secondary shock
in the HI-cavity drilled by the jet. The particle acceleration at the shock can
be the origin of radio and X-ray filaments observed near the eastern edge of
W50 nebula surrounding the galactic jet source SS433.
