Pair-instability supernovae via collision runaway in young dense star clusters

Stars with helium cores between similar to 64 and 133 M? 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 similar to 2 x 10(-9) Mpc-3 yr-1 of the two observed pair-instability supernovae, SN 2007bi and PTF 10nmn, neither of which has 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. We estimate that the telescope will observe similar to 102 such events per year that originate from the collisional runaway mergers in clusters.