ON THE ONSET OF SECONDARY STELLAR GENERATIONS IN GIANT STAR-FORMING REGIONS AND MASSIVE STAR CLUSTERS

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Here we consider the strong evolution experienced by the matter reinserted by massive stars, both in giant star forming regions driven by a constant star formation rate, and in massive and coeval superstar clusters. In both cases we take into consideration the changes induced by stellar evolution on the number of massive stars, the number of ionizing photons and the integrated mechanical luminosity of the star forming regions. The latter is at all times compared with the critical luminosity that defines, for a given size, the lower mechanical luminosity limit above which the matter reinserted via strong winds and supernova explosions suffers frequent and recurrent thermal instabilities that reduce its temperature and pressure and inhibit its exit as part of a global wind. Instead, the unstable reinserted matter is compressed by the pervasive hot gas, and photoionization maintains its temperature at T $\sim$ 10$^4$ K. As the evolution proceeds, more unstable matter accumulates and the unstable clumps grow in size. Here we evaluate the possible self-shielding of thermally unstable clumps against the UV radiation field. Self shielding allows for a further compression of the reinserted matter which rapidly develops a high density neutral core able to absorb in its outer skin the incoming UV radiation. Under such conditions the cold (T $\sim$ 10 K) neutral cores soon surpass the Jeans limit and become gravitationally unstable, causing a new stellar generation with the matter reinserted by former massive stars. We present the results of several calculations of this positive star formation feedback scenario promoted by strong radiative cooling and mass loading.