  A number of giant cometary HII regions (GCH) sheathed inside molecular bow
shocks (MBS) are found along spiral arms of the barred galaxy M83. The open
cone structure is explained by a model of expanded HII front in a gaseous arm
with steep density gradient, and the bow shock is shown to be formed by
encounter of an HII region with the supersonic gas flow across the arm. It is
suggested that dual-side compression of molecular gas at the bow head between
the MBS and GCH enhances star formation along the spiral arms.
