On the stability of very massive primordial stars

The stability of metal-free very massive stars (Z = 0; M = 120-500 M-circle dot) is analyzed and compared with metal-enriched stars. Such zero-metallicity stars are unstable to nuclear-powered radial pulsations on the main sequence, but the growth timescale for these instabilities is much longer than for their metal-rich counterparts. Since they stabilize quickly after evolving off the zero-age main sequence (ZAMS), the pulsation may not have sufficient time to drive appreciable mass loss in Z = 0 stars. For reasonable assumptions regarding the efficiency of converting pulsational energy into mass loss, we find that, even for the larger masses considered, the star may die without losing a large fraction of its mass. We find a transition between the epsilon- and kappa -mechanisms for pulsational instability at Z similar to 2 x 10(-4) to 2 x 10(-3). For the most metal-rich stars, the kappa -mechanism yields much shorter e-folding times, indicating the presence of a strong instability. We thus stress the fundamental difference of the stability and late stages of evolution between very massive stars born in the early universe and those that might be born today.