Dynamical parallax of σ Ori AB : mass , distance and age

The massive OB-type binary σ Ori AB is in the centre of the very young σ Orionis cluster. I have computed the most probable distances and masses of the binary for several ages using a dynamical parallax-like method. It incorporates the BV RIH-band apparent magnitudes of both components, precise orbital parameters, interstellar extinction and a widely used grid of stellar models from the literature, the Kepler’s third law and a χ minimisation. The derived distance is 334 −22 pc for an age of 3±2Ma; larger ages and distances are unlikely. The masses of the primary and the secondary lie on the approximate intervals 16–20 and 10–12M⊙, respectively. I also discuss the possibility of σ Ori AB being a triple system at ∼385pc. These results will help to constrain the properties of young stars and substellar objects in the σ Orionis cluster.