Abstract

There are two known mechanisms of triggered star formation associated with HII regions. One is the collect-and-collapse process of the shell accumulated around an expanding HII region, and the other is radiation-driven implosion (RDI) of bright-rimmed clouds (BRCs) originated from pre-existing cloud clumps. They are very briefly reviewed first. We then present the main results of our recent observations on the RDI star formation in BRCs. Finally, a third possible mechanism of triggering is suggested, which is attributed to the formation of elephant trunk-like structures due to hydrodynamical instability of ionization/shock fronts.