Evolution of Collisionally Merged Massive Stars

We investigate the evolution of collisionally merged stars with mass of ~100MSun which might be formed in dense star clusters. We assumed that massivestars with several tens Msun collide typically after ~1Myr of the formation ofthe cluster and performed hydrodynamical simulations of several collisionevents. Our simulations show that after the collisions, merged stars haveextended envelopes and their radii are larger than those in the thermalequilibrium states and that their interiors are He-rich because of the stellarevolution of the progenitor stars. We also found that if the mass-ratio ofmerging stars is far from unity, the interior of the merger product is not wellmixed and the elemental abundance is not homogeneous. We then followed theevolution of these collision products by a one dimensional stellar evolutioncode. After an initial contraction on the Kelvin-Helmholtz (thermal adjustment)timescale (~1000-10000 yr), the evolution of the merged stars traces that ofsingle homogeneous stars with corresponding masses and abundances, while theinitial contraction phase shows variations which depend on the mass ratio ofthe merged stars. We infer that, once runaway collisions have set in,subsequent collisions of the merged stars take place before mass loss bystellar winds becomes significant. Hence, stellar mass loss does not inhibitthe formation of massive stars with mass of ~1000Msun.Comment: 11 pages, 16 figures embedded, submitted to Ap