Interpretation of the variability of the Cephei star Scorpii^*,**

I. The multiple character

¹ Institute of Astronomy, Catholic University Leuven, Celestijnenlaan 200 B, 3001 Leuven, Belgium e-mail: katrien.uytterhoeven@ster.kuleuven.ac.be
² Mercator Telescope, Calle Alvarez de Abreu 70, 38700 Santa Cruz de La Palma, Spain
³ Department of Physics and Astronomy, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208, USA
⁴ Department of Physics and Astronomy, The Open University, Milton Keynes, MK7 6AA, UK
⁵ Department of Astrophysics, University of Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands
⁶ Nordic Optical Telescope, Apartado de Correos 474, 38700 Santa Cruz de La Palma, Spain

Received: 4 May 2004
Accepted: 15 July 2004

Abstract

We derive accurate values of the orbital parameters of the close binary β Cephei star λ Scorpii. Moreover, we present the first determination of the properties of the triple system to which λ Scorpii belongs. Our analysis is based on a time series of 815 high-resolution spectra, covering a timespan of 14 years. We find a close orbit of 59525 days () and a wide orbit of approximately 1082^d days (). The orbital parameters of the triple star and a spectrum synthesis lead us to conclude that the system is composed of two early-type B stars and a low-mass pre-main-sequence star rather than containing an ultra-massive white dwarf as claimed before. Our proposed configuration is compatible with population synthesis. The radial velocity variations of the primary allow us to confirm the presence of at least one pulsation mode with frequency 4.679410 c d^-1 which is subject to the light-time effect in the triple system. A detailed analysis of the complex line-profile variations is described in a subsequent paper.