2002A&A...394L..43K

We present an H-band image of the companion of χ¹ Orionis taken with the Keck adaptive optic system and NIRC 2 camera equipped with a 300mas-diameter coronographic mask. The direct detection of this companion star enables us to calculate dynamical masses using only Kepler's laws (M_A=1.01±0.13M_☉, M_B=0.15±0.02M_☉), and to study stellar evolutionary models at a wide spread of masses. The application of Baraffe et al. (1998A&A...337..403B) pre-main-sequence models implies an age of 70-130Myrs. This is in conflict to the age of the primary, a confirmed member of the Ursa Major Cluster with a canonical age of 300Myrs. As a consequence, either the models at low masses underestimate the age or the Ursa Major Cluster is considerably younger than assumed.