Dynamical expansion of ionization and dissociation front around a massive star: A starburst mechanism

We quantitatively examine the significance of star formation triggered in the swept-up shell around an expanding H II region. If the swept-up molecular gas is sufficiently massive, new OB stars massive enough to repeat the triggering process will form in the shell. We determine the lower limit (M-thr) for the mass of the star that sweeps up the molecular gas, where at least one new star with mass M-* > M-thr forms after shell fragmentation. To calculate the threshold stellar mass, we examine how massive molecular shells can form around various central stars, by performing detailed numerical radiation hydrodynamics calculations. The mass of the photodissociated gas is generally larger than the mass of the photoionized gas. However, the swept-up molecular mass exceeds the photodissociated mass with a higher mass star of. The accumulated molecular mass generally increases M-* greater than or similar to 20 M-circle dot, with the stellar mass, and amounts to 10(4)-10(5) M circle dot for M* with an ambient density of n similar to 10(2) cm(-3). The threshold stellar mass is with a star formation efficiency of epsilon similar to 0.1 and n similar to 10(2). We examine the generality of this mode of runaway triggering for different sets of parameters and find that M-thr similar to 15-20 M-circle dot in various situations. If the ambient density is too high or the star formation efficiency is too low, the 15-20 M-circle dot triggering is not runaway, but a single event. d