The VLT-FLAMES Tarantula Survey

XXII. Multiplicity properties of the B-type stars^⋆,⋆⋆

¹ Astrophysics Research Centre, School of Mathematics & Physics, Queen’s University Belfast, BT7 1NN, UK
e-mail: P.Dufton@qub.ac.uk
² ESA/Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
³ UK Astronomy Technology Centre, Royal Observatory Edinburgh, Blackford Hill, Edinburgh, EH9 3HJ, UK
⁴ Dept. of Physics & Astronomy, University College London, Gower Street, London, WC1E 6BT, UK
⁵ Instituto de Astrofísica de Canarias, 38205, La Laguna, Tenerife, Spain
⁶ Departamento de Astrofísica, Universidad de La Laguna, 38205, La Laguna, Tenerife, Spain
⁷ Astronomical Institute Anton Pannekoek, Amsterdam University, Science Park 904, 1098 XH, Amsterdam, The Netherlands
⁸ Argelander-Institut für Astronomie der Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
⁹ Departamento de Astrofísica, Centro de Astrobiología (INTA-CSIC), Campus ESA, Apartado Postal 78, 28 691 Villanueva de la Cañada, Madrid, Spain

Received: 26 March 2015
Accepted: 21 May 2015

Abstract

We investigate the multiplicity properties of 408 B-type stars observed in the 30 Doradus region of the Large Magellanic Cloud with multi-epoch spectroscopy from the VLT-FLAMES Tarantula Survey (VFTS). We use a cross-correlation method to estimate relative radial velocities from the helium and metal absorption lines for each of our targets. Objects with significant radial-velocity variations (and with an amplitude larger than 16 km s^-1) are classified as spectroscopic binaries. We find an observed spectroscopic binary fraction (defined by periods of <10^3.5 d and mass ratios >0.1) for the B-type stars, f_B(obs) = 0.25 ± 0.02, which appears constant across the field of view, except for the two older clusters (Hodge 301 and SL 639). These two clusters have significantly lower binary fractions of 0.08 ± 0.08 and 0.10 ± 0.09, respectively. Using synthetic populations and a model of our observed epochs and their potential biases, we constrain the intrinsic multiplicity properties of the dwarf and giant (i.e. relatively unevolved) B-type stars in 30 Dor. We obtain a present-day binary fraction f_B(true) = 0.58 ± 0.11, with a flat period distribution. Within the uncertainties, the multiplicity properties of the B-type stars agree with those for the O stars in 30 Dor from the VFTS.