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Sequential formation of subgroups in OB associations
Elmegreen, B. G.; Lada, C. J.
AA(Harvard College Observatory and Smithsonian Astrophysical Observatory, Cambridge, Mass.), AB(Harvard College Observatory and Smithsonian Astrophysical Observatory, Cambridge, Mass.)
Astrophysical Journal, Part 1, vol. 214, June 15, 1977, p. 725-741. (ApJ Homepage)
Publication Date:
NASA/STI Keywords:
B Stars, Lyman Spectra, Nebulae, O Stars, Star Clusters, Stellar Evolution, Interstellar Gas, Protostars, Shock Fronts, Stellar Models, Stellar Structure
Bibliographic Code:


It is noted that the Lyman continuum radiation from a cluster of OB stars will drive ionization (I) fronts into nearby neutral material and that an I front should be preceeded by a shock (S) front as it moves into an adjacent molecular cloud near dense nebulae. The gravitational stability of the high-density layer of neutral gas that accumulates between the I and S fronts is investigated to determine whether star formation is likely to occur there. The results show that the shocked neutral layer will become gravitationally unstable after several million years for I-S fronts propagating into molecular clouds of moderate density and that the stars which may form as the layer collapses are likely to be more massive than those which form in unshocked remote parts of the same molecular cloud. It is found that if the stars which eventually form in the shocked layer are OB stars, a new system of I-S fronts will propagate into the remaining cloud after these stars reach the main sequence, and another cycle of OB star formation will be initiated. It is therefore proposed that massive OB stars may be formed in sequential bursts during the lifetime of some large molecular clouds.

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