Making a Supermassive Star by Stellar Bombardment

Approximately 200 supermassive black holes (SMBHs) have been discovered within the first ∼gigayear after the Big Bang. One pathway for the formation of SMBHs is through the collapse of supermassive stars (SMSs). A possible obstacle to this scenario is that the collapsing gas fragments and forms a cluster of main-sequence stars. Here, we raise the possibility that stellar collisions may be sufficiently frequent and energetic to inhibit the contraction of the massive protostar, avoiding strong UV radiation driven outflows, and allowing it to continue growing into an SMS. We investigate this scenario with semianalytic models incorporating star formation; gas accretion; dynamical friction from stars and gas; stellar collisions; and gas ejection. We find that when the collapsing gas fragments at a density of ≲3 × 10¹⁰ cm^-3, the central protostar contracts due to infrequent stellar mergers, and in turn photoevaporates the remaining collapsing gas, resulting in the formation of a ≲10⁴ M_⊙ object. On the other hand, when the collapsing gas fragments at higher densities (expected for a metal-poor cloud with Z ≲ 10^-5 Z_⊙ with suppressed H₂ abundance) the central protostar avoids contraction and keeps growing via frequent stellar mergers, reaching masses as high as ∼10⁵-10⁶ M_⊙. We conclude that frequent stellar mergers represent a possible pathway to form massive BHs in the early universe.