The VLT-FLAMES Tarantula Survey

X. Evidence for a bimodal distribution of rotational velocities for the single early B-type stars ^⋆

¹ Astrophysics Research Centre, School of Mathematics and Physics, Queen’s University Belfast, Belfast, BT7 1NN UK
e-mail: p.dufton@qub.ac.uk
² Argelander Institut für Astronomie der Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
³ UK Astronomy Technology Centre, Royal Observatory Edinburgh, Blackford Hill, Edinburgh, EH9 3HJ, UK
⁴ University of Vienna, Department of Astronomy, Türkenschanzstr. 17, 1180 Vienna, Austria
⁵ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁶ Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218, USA
⁷ Department of Physics and Astronomy, University College London, Gower Street, London, WC1E 6BT, UK
⁸ Institute of Astronomy, The Observatories, Madingley Road, Cambridge CB3 0HA, UK
⁹ Astronomical Institute “Anton Pannekoek”, University of Amsterdam, Postbus 94249, 1090 GE, Amsterdam, The Netherlands
¹⁰ Instituto de Astrofísica de Canarias, 38200, La Laguna, Tenerife, Spain
¹¹ Departamento de Astrofísica, Universidad de La Laguna, 38205, La Laguna, Tenerife, Spain
¹² Scottish Universities Physics Alliance, Institute for Astronomy, University of Edinburgh, Royal Observatory Edinburgh, Blackford Hill, Edinburgh, EH9 3HJ, UK
¹³ Armagh Observatory, College Hill, Armagh BT61 9DG, UK

Received: 22 August 2012
Accepted: 2 December 2012

Abstract

Aims. Projected rotational velocities (v_esini) have been estimated for 334 targets in the VLT-FLAMES Tarantula Survey that do not manifest significant radial velocity variations and are not supergiants. They have spectral types from approximately O9.5 to B3. The estimates have been analysed to infer the underlying rotational velocity distribution, which is critical for understanding the evolution of massive stars.

Methods. Projected rotational velocities were deduced from the Fourier transforms of spectral lines, with upper limits also being obtained from profile fitting. For the narrower lined stars, metal and non-diffuse helium lines were adopted, and for the broader lined stars, both non-diffuse and diffuse helium lines; the estimates obtained using the different sets of lines are in good agreement. The uncertainty in the mean estimates is typically 4% for most targets. The iterative deconvolution procedure of Lucy has been used to deduce the probability density distribution of the rotational velocities.

Results. Projected rotational velocities range up to approximately 450 km s^-1 and show a bi-modal structure. This is also present in the inferred rotational velocity distribution with 25% of the sample having 0 ≤ v_e ≤ 100 km s^-1 and the high velocity component having v_e ~ 250 km s^-1. There is no evidence from the spatial and radial velocity distributions of the two components that they represent either field and cluster populations or different episodes of star formation. Be-type stars have also been identified.

Conclusions. The bi-modal rotational velocity distribution in our sample resembles that found for late-B and early-A type stars. While magnetic braking appears to be a possible mechanism for producing the low-velocity component, we can not rule out alternative explanations.