On the Upper Mass Limit for Main-Sequence Stars

Two aspects of the problem of the upper mass limit are investigated. First, a linear nonadiabatic analysis is used to investigate the dependence of this limit on composition. Second, a nonlinear hydrodynamic calculation has been carried out to study the consequences for a star whose mass exceeds the limit. The maximum stable mass is not found to be a sensitive function of composition. This mass, , decreases with increasing F at fixed Z; at fixed F, decreases with decreasing Z. In the hydrodynamic calculation it was necessary to increase artificially the very slow growth rate of the pulsations. After initiation of small-amplitude pulsations in the fundamental mode, the pulsations grew to a limiting amplitude too small to produce mass loss. It is shown that the artificial increase in growth rate probably greatly underestimates the shock-wave dissipation. If this is true, mass loss may never occur. Instead, stars with masses above the stability limit may simply pulsate at some (perhaps small) amplitude until nuclear evolution stabilizes them.