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

We analyze 4,859 O-stars in the OGLE-III 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 flogP ∝ (logP)^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 vₚᵣₒⱼ $<$ 24.5 km s⁻¹) and field runaways (vₚᵣₒⱼ $>$ 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.