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 (Mthr) for the mass of the star that sweeps up the molecular gas, where at least one new star with mass M∗ > Mthr forms after the shell fragmentation. To calculate the threshold stellar mass, Mthr, 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 M∗ & 20 M⊙. The accumulated molecular mass generally increases with the stellar mass, and amounts to 10 M⊙ for M∗ & 20 M⊙ with an ambient density of n ∼ 10 2 cm. The threshold stellar mass is Mthr ∼ 18M⊙ with the star-formation efficiency of ǫ ∼ 0.1 and n ∼ 10 2 cm. We examine the generality of this mode of run-away triggering for different sets of parameters, and found that Mthr ∼ 15− 20 M⊙ in various situations. If the ambient density is too high or the star-formation efficiency is too low, the triggering is not run-away, but a single event. Subject headings: Circumstellar matter – H II regions – ISM: molecules – STARS : formation