Pair-Instability Supernovae via Collision Runaway in Young Dense Star   Clusters
  Stars with helium cores between ~64 and 133 M_sun are theoretically predicted to die as pair-instability supernovae. This requires very massive progenitors, which are theoretically prohibited for Pop II/I stars within the Galactic stellar mass limit due to mass loss via line-driven winds. However, the runaway collision of stars in a dense, young star cluster could create a merged star with sufficient mass to end its life as a pair-instability supernova, even with enhanced mass loss at non-zero metallicity. We show that the predicted rate from this mechanism is consistent with the inferred volumetric rate of roughly ~2x10^-9 Mpc^-3 yr^-1 of the two observed pair-instability supernovae, SN 2007bi and PTF 10nmn, neither of which have metal-free host galaxies. Contrary to prior literature, only pair-instability supernovae at low redshifts z<2 will be observable with the Large Synoptic Survey Telescope (LSST). We estimate the telescope will observe ~10^2 such events per year that originate from the collisional runaway mergers in clusters. 
Tony Pan, Abraham Loeb, Daniel Kasen
astro-ph.HE