The VLT-FLAMES Tarantula Survey^⋆,⋆⋆

XII. Rotational velocities of the single O-type stars

¹ Astronomical Institute Anton Pannekoek, Amsterdam University, Science Park 904, 1098 XH Amsterdam, The Netherlands
e-mail: o.h.ramirezagudelo@uva.nl
² Instituto de Astrofísica de Canarias, C/ vía Láctea s/n, 38200 La Laguna, Tenerife, Spain
³ Departamento de Astrofísica, Universidad de La Laguna, Avda. Astrofísico Francisco Sánchez s/n, 38071 La Laguna, Tenerife, Spain
⁴ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore MD 21218, USA
⁵ Instituut voor Sterrenkunde, Universiteit Leuven, Celestijnenlaan 200 D, 3001 Leuven, Belgium
⁶ Observatories of the Carnegie Institution for Science, 813 Santa Barbara St, Pasadena CA 91101, USA
⁷ Cahill Center for Astrophysics, California Institute of Technology, Pasadena CA 91125, USA
⁸ Astrophysics Research Centre, School of Mathematics and Physics, Queen’s University of Belfast, Belfast BT7 1NN, UK
⁹ Armagh Observatory, College Hill, Armagh BT61 9DG, Northern Ireland, UK
¹⁰ UK Astronomy Technology Centre, Royal Observatory Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK
¹¹ Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
¹² European Space Astronomy Centre (ESAC), Camino bajo del Castillo s/n, Urbanizacion Villafranca del Castillo, Villanueva de la Cañada, E-28692 Madrid, Spain
¹³ Instituto de Astrofísica de Andalucía-CSIC, Glorieta de la Astronomía s/n, 18008 Granada, Spain
¹⁴ Institute of Astronomy with NAO, Bulgarian Academy of Science, PO Box 136, 4700 Smoljan, Bulgaria
¹⁵ Centro de Astrobiología (CSIC-INTA), Ctra. de Torrejón a Ajalvir km-4, 28850 Torrejón de Ardoz, Madrid, Spain
¹⁶ Universitäts-Sternwarte München, Scheinerstrasse 1, 81679 München, Germany

Received: 29 May 2013
Accepted: 10 September 2013

Abstract

Context. The 30 Doradus (30 Dor) region of the Large Magellanic Cloud, also known as the Tarantula nebula, is the nearest starburst region. It contains the richest population of massive stars in the Local Group, and it is thus the best possible laboratory to investigate open questions on the formation and evolution of massive stars.

Aims. Using ground-based multi-object optical spectroscopy obtained in the framework of the VLT-FLAMES Tarantula Survey (VFTS), we aim to establish the (projected) rotational velocity distribution for a sample of 216 presumably single O-type stars in 30 Dor. The sample is large enough to obtain statistically significant information and to search for variations among subpopulations – in terms of spectral type, luminosity class, and spatial location – in the field of view.

Methods. We measured projected rotational velocities, ν_esini, by means of a Fourier transform method and a profile fitting method applied to a set of isolated spectral lines. We also used an iterative deconvolution procedure to infer the probability density, P(ν_e), of the equatorial rotational velocity, ν_e.

Results. The distribution of ν_esini shows a two-component structure: a peak around 80 kms^-1 and a high-velocity tail extending up to ~600 kms^-1. This structure is also present in the inferred distribution P(ν_e) with around 80% of the sample having 0 < ν_e ≤ 300 kms^-1 and the other 20% distributed in the high-velocity region. The presence of the low-velocity peak is consistent with what has been found in other studies for late O- and early B-type stars.

Conclusions. Most of the stars in our sample rotate with a rate less than 20% of their break-up velocity. For the bulk of the sample, mass loss in a stellar wind and/or envelope expansion is not efficient enough to significantly spin down these stars within the first few Myr of evolution. If massive-star formation results in stars rotating at birth with a large portion of their break-up velocities, an alternative braking mechanism, possibly magnetic fields, is thus required to explain the present-day rotational properties of the O-type stars in 30 Dor. The presence of a sizeable population of fast rotators is compatible with recent population synthesis computations that investigate the influence of binary evolution on the rotation rate of massive stars. Even though we have excluded stars that show significant radial velocity variations, our sample may have remained contaminated by post-interaction binary products. That the high-velocity tail may be populated primarily (and perhaps exclusively) by post-binary interaction products has important implications for the evolutionary origin of systems that produce gamma-ray bursts.