Evolution of Collisionally Merged Massive Stars

Abstract

We investigate the evolution of collisionally merged stars with masses of ~100 M☉ which might be formed in dense star clusters. We assumed that massive stars with several tens of M☉ collide typically after ~1 Myr of the formation of the cluster and performed hydrodynamical simulations of several collision events. Our simulations show that after the collisions merged stars have extended envelopes and their radii are larger than those in the thermal equilibrium states and that their interiors are He-rich because of the stellar evolution of the progenitor stars. We also found that if the mass ratio of merging stars is far from unity, the interior of the merger product is not well mixed, and the elemental abundance is not homogeneous. We then followed the evolution of these collision products with a one-dimensional stellar evolution code. After an initial contraction on the Kelvin-Helmholtz (thermal adjustment) timescale (~103-104 yr), the evolution of the merged stars traces that of single homogeneous stars with corresponding masses and abundances, while the initial contraction phase shows variations which depend on the mass ratio of the merged stars. We infer that once runaway collisions have set in, subsequent collisions of the merged stars taking place before mass loss by stellar winds become significant. Hence, stellar mass loss does not inhibit the formation of massive stars with masses of ~1000 M☉.