The Close Binary Properties of Massive Stars across Different Environments within the LMC

Abstract We analyze 4859 O stars in the third phase of the Optical Gravitational Lensing Experiment photometric survey of the LMC, including 415 eclipsing binaries (EBs). After accounting for the geometrical probability of eclipses, the period distribution of O-type binaries across P = 2.5–200 days follows a power law f log P ∝ (log P ) Π with Π = −0.34 ± 0.06, which is skewed toward shorter periods compared to Opik’s law (Π = 0). We divide our O stars into seven environments based on their clustering with B stars and other O stars. The EB fraction of O stars in young clusters is 10.2% ± 0.6%, which matches the 10.8% ± 2.1% for O stars in young Milky Way clusters. O stars in old clusters exhibit a lower EB fraction of 5.5% ± 0.9% due to the effects of binary evolution. O stars in young dense clusters, young sparse associations, and even low-mass clusters that formed in situ in the field have similar EB fractions. This uniformity suggests that the formation of close massive binaries depends on small-scale gas physics, e.g., fragmentation and migration within protostellar disks, whereas N -body interactions that scale with cluster density do not affect the close binary properties of massive stars that remain in clusters. Conversely, ejected O stars in the field exhibit a lower close binary fraction. The EB fractions of field walkaways (projected velocities v proj 24.5 km s −1 ) are 7.3% ± 1.0% and 4.7% ± 1.0%, respectively. These values suggest that most field O stars were dynamically ejected via N -body interactions from their birth clusters, whereas field O stars that formed in situ or were kicked from supernova explosions in binaries contribute 17% and <28%, respectively, to the field population.