Orbits, Distance, and Stellar Masses of the Massive Triple Star Σ Orionis

We present interferometric observations of the σ Orionis triple system using the CHARA Array, NPOI, and VLTI. Using these measurements, we spatially resolve the orbit of the close spectroscopic binary (Aa,Ab) for the first time and present a revised orbit for the wide pair (A,B). Combining the visual orbits with previously published radial velocity measurements and new radial velocities measured at CTIO, we derive dynamical masses for the three massive stars in the system of MAa=16.99±0.20  M , MAb=12.81±0.18  M , and MB=11.5±1.2  M . The inner and outer orbits in the triple are not coplanar, with a relative inclination of  120 –127. The orbital parallax provides a precise distance of 387.5±1.3 pc to the system. This is a significant improvement over previous estimates of the distance to the young σ Orionis cluster.