Lunch talk on Apr. 14, 2026

Comprehensive population synthesis predictions for massive binary stars in the Small Magellanic Cloud

Speaker: Xiao-Tian Xu (SJTU)

Venue: SWIFAR Building 2111

Time: 12:45 PM, Tuesday, Apr. 13, 2026

Abstract: 

Past and ongoing gravitational wave detections have fostered a wide interest in understanding the formation of binary black holes (BBHs). Several formation scenarios have been proposed, including the evolution of isolated massive binaries. However, most of the observed merging black holes are at cosmological distances, and thus likely at low metallicity, where individual stars are not observable. The Small Magellanic Cloud (SMC) provides a unique laboratory to probe the binary evolution scenario thanks to its rich massive star population and low metallicity. In this work, we present comprehensive population synthesis predictions for the massive binary stars at the SMC metallicity throughout all their different evolutionary stages, based on a dense grid of more than 50,000 detailed binary evolution models. Our models well reproduce the observed Wolf-Rayet (WR) star + main sequence (MS) star binaries in the SMC (7 predicted), and we predict ~200 BH+MS star binaries, mostly associated with Be/Oe stars, and 6% of the observed SMC O stars are expected to have BH companions. Then, the BH+MS binaries, if massive enough, may form BH+WR binaries. By comparing with the observed stellar population in the SMC, we have detected many discrepancies between observations and theoretical predictions, calling for revision of the current theoretical framework of massive binary evolution. In particular, the absence of long-period WR/BH+OB star binaries challenges the role of the common envelope evolution in the formation of merging BBHs.