The VLT-FLAMES Tarantula Survey

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, e sin i, 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( e ), of the equatorial rotational velocity, e . Results. The distribution of e sin i shows a two-component structure: a peak around 80 km s −1 and a high-velocity tail extending up to ∼600 km s −1 . This structure is also present in the inferred distribution P( e ) with around 80% of the sample having 0 < e ≤ 300 km s −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 highvelocity 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.

The VLT-FLAMES Tarantula Survey

“…For stars near the Eddington limit, the initial rotational velocity becomes critical. We note that Dufton et al (2013) and Ramírez-Agudelo et al (2013) predicted targets with a wide range of rotational velocities including v o > ∼ 400 km s −1 for their B-type and O-type non-supergiant VFTS samples, respectively. As the original rotational velocities of our supergiants are currently unknown, the implied evolutionary masses must be considered with considerable caution.…”

“…We have used the grid of models of Brott et al (2011) as these include a metallicity appropriate to the LMC, and cover a wide range of initial equatorial velocities (0 ≤ v o < ∼ 600 km s −1 ) and masses (0 ≤ M o ≤ 60 M ); they have also been used in other papers in this series, thereby ensuring consistency. We have chosen to use those models with v o 225 km s −1 as this was compatible with the mean equatorial velocity inferred for (near) main sequence stars observed in the Tarantula survey (Dufton et al 2013;Ramírez-Agudelo et al 2013). The uncertainties arising from this choice will be discussed below.…”

The very massive star content of the nuclear star clusters in NGC 5253

“…These two effects lead to a higher ionizing photon output in the Lyman continuum at ages ≥ 4 Myr. However, measurements of the rotational velocities of single O-type stars in the 30 Dor region show that they rotate with a rate of less than 20% of their breakup velocity (Ramírez-Agudelo et al 2013). Thus the ionizing outputs of the Leitherer et al (2014) models are likely to be over-estimates.…”

The VLT-FLAMES Tarantula survey

“…Investigation of the single O-type stars revealed that the distribution of their rotational velocities has a two-component structure (Ramírez-Agudelo et al 2013), comprising a low velocity peak (∼80 km s −1 ) and a high-velocity tail (extending to ∼600 km s −1 ). The origin of the high velocity cohort is currently uncertain, with the relative contribution from natal and binary-driven channels still ill-constrained Ramírez-Agudelo et al 2013).…”

“…Investigation of the single O-type stars revealed that the distribution of their rotational velocities has a two-component structure (Ramírez-Agudelo et al 2013), comprising a low velocity peak (∼80 km s −1 ) and a high-velocity tail (extending to ∼600 km s −1 ). The origin of the high velocity cohort is currently uncertain, with the relative contribution from natal and binary-driven channels still ill-constrained Ramírez-Agudelo et al 2013). With a projected equatorial rotational velocity, v e sin i ∼ 334 ± 18 km s −1 (Ramírez-Agudelo et al 2013), the O9 IIIn star VFTS 399 is a member of this group (Walborn et al 2014), while its location on the periphery of the 30 Dor complex (Fig.…”