The VLT-FLAMES Tarantula Survey XII. Rotational velocities of the single O-type stars

TL;DR

This study measures the rotational velocities of 216 single O-type stars in 30 Doradus, revealing a bimodal distribution with most stars rotating slowly and a significant fraction rotating rapidly, which has implications for stellar evolution and gamma-ray burst progenitors.

Contribution

First detailed measurement of projected rotational velocities for a large sample of presumably single O-type stars in 30 Doradus, using Fourier transform and profile fitting methods.

Findings

Distribution shows a peak around 80 km/s and a high-velocity tail up to 600 km/s.

Approximately 80% of stars have equatorial velocities ≤300 km/s.

Presence of fast rotators suggests binary interactions influence stellar rotation.

Abstract

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 Doradus (30 Dor). Methods. We measured projected rotational velocities, \vrot, by means of a Fourier transform method and a profile fitting method applied on a set of isolated spectral lines. We also used an iterative deconvolution procedure to infer the probability density, $\rm{P(\veq)}$, of the equatorial rotational velocity, \veq. Results. The distribution of \vrot\ shows a two-component structure: a peak around 80 \kms\ and a high-velocity tail extending up to $\sim$600 \kms. This structure is also present in the inferred distribution $\rm{P(\veq)}$ with around 80% of the sample having 0 $<$ \veq\, $\leq\, 300$ \kms\ and the other 20% distributed in the high-velocity region. 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. 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. Despite the fact that we have excluded stars that show significant radial velocity variations, our sample may have remained contaminated by post-interaction binary products. The fact that the high-velocity tail may be preferentially (and perhaps even exclusively), populated by post-binary interaction products, has important implications for the evolutionary origin of systems that produce gamma-ray bursts.