Merger Conditions of Population III Protostar Binaries

Abstract Massive close binary stars with extremely small separations have been observed, and they are possible progenitors of gravitational-wave sources. The evolution massive binaries in the protostellar accretion stage is key to understanding their formation process. We, therefore, investigate how protostars, consisting a high-density core vast low-density envelope, can approach each other but not coalesce. To coalescence conditions, we conduct smoothed particle hydrodynamics simulations following equal-mass different initial separations. Since Population (Pop) I III protostars similar interior structures, adopt specific Pop model mass radius 7.75 M ⊙ 61.1 R obtained by stellar calculations. Our results show that separation decreases due transport orbital angular momentum spin momentum. If less than about 80% sum radius, coalesces time shorter tidal lock timescale. loss up merging ≲3%. After coalescence, star rotates rapidly, its structure independent separation. We conclude there must be some shrinking mechanism after contract enter zero-age main-sequence stage.