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Issue A&A
Volume 394, Number 3, November II 2002
Page(s) L43 - L46
Section Letters
DOI http://dx.doi.org/10.1051/0004-6361:20021377



A&A 394, L43-L46 (2002)
DOI: 10.1051/0004-6361:20021377

Letter

Direct detection of the companion of $\chi^1$ Orionis

B. König1, K. Fuhrmann1, R. Neuhäuser1, D. Charbonneau2 and R. Jayawardhana3

1  Max-Planck-Institut für extraterrestrische Physik, Gießenbachstraße 1, 85748 Garching, Germany
2  California Institute of Technology, 105-24 (Astronomy) 1200 E. California Blvd., Pasadena, CA 91125, USA
3  Department of Astronomy, University of Michigan, Ann Arbor, MI 48109, USA

(Received 28 August 2002 / Accepted 18 September 2002)

Abstract
We present an H-band image of the companion of $\chi^1$ Orionis taken with the Keck adaptive optic system and NIRC 2 camera equipped with a 300 mas-diameter coronographic mask. The direct detection of this companion star enables us to calculate dynamical masses using only Kepler's laws ( $M_{\rm A} =1.01\pm0.13~M_{\odot}$, $M_{\rm B} =0.15\,\pm\,0.02~M_{\odot}$ ), and to study stellar evolutionary models at a wide spread of masses. The application of Baraffe et al. (1998) pre-main-sequence models implies an age of 70-130 Myrs. This is in conflict to the age of the primary, a confirmed member of the Ursa Major Cluster with a canonical age of 300 Myrs. As a consequence, either the models at low masses underestimate the age or the Ursa Major Cluster is considerably younger than assumed.


Offprint request: B. König, bkoenig@mpe.mpg.de

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