On the Stability of Very Massive Primordial Stars

The stability of metal-free very massive stars ($Z$ = 0; $M = 120 - 500\msol$) is analyzed and compared with metal-enriched stars. Such zero-metalstars are unstable to nuclear-powered radial pulsations on the main sequence,but the growth time scale for these instabilities is much longer than for theirmetal-rich counterparts. Since they stabilize quickly after evolving off theZAMS, the pulsation may not have sufficient time to drive appreciable mass lossin Z = 0 stars. For reasonable assumptions regarding the efficiency ofconverting pulsational energy into mass loss, we find that, even for the largermasses considered, the star may die without losing a large fraction of itsmass. We find a transition between the $\epsilon$- and $\kappa$-mechanisms forpulsational instability at $Z\sim 2\E{-4} - 2\E{-3}$. For the most metal-richstars, the $\kappa$-mechanism yields much shorter $e$-folding times, indicatingthe presence of a strong instability. We thus stress the fundamental differenceof the stability and late stages of evolution between very massive stars bornin the early universe and those that might be born today.Comment: 7 pages, 5 figures. Minor changes, more results given in Table 1, accepted for publication in Ap