NONCOEVAL STAR-FORMATION, STARBURSTS, AND THE GROWTH OF SUPERSHELLS IN OB ASSOCIATIONS

There is strong evidence that the epoch of massive-star formation in OB associations is spread over intervals of 10-20 Myr as a result of subclustering, continuous or sequential star formation, or delayed collapse of the most massive stars. The growth of supershells is strongly influenced by this star formation history and by mass deposition to the hot bubble interior (''radiative poisoning''). Even in cases of coeval or continuous star formation, the rising energy input from massive stars, as they evolve off the main sequence, may be observed as a ''starburst'' with peak energy deposition 3-10 times that of the late-time rate due to supernovae and an excess of Wolf-Rayet stars (WR/O ratio approximate to 0.1). The peak in energy deposition results in faster shell growth and velocity evolution than in constant-luminosity models. At finite bubble ages, this mechanism can alter the covering factor of superbubbles in the disk by factors of 2-3 and can accelerate the dynamical effects of galactic ''superwinds.'' In contrast, if sufficient matter is entrained in the hot bubble, radiative poisoning will stall its growth.