The VLT-FLAMES Tarantula Survey

Abstract

Aims. Projected rotational velocities (v e sin i) 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.

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.…”

“…The data reduction followed the procedures discussed in Dufton et al (2013) and only a brief description is given here. For stars with ∆RV ≤ 30 km s −1 , the LR02 spectra from all useable exposures were combined using either a median or weighted σ-clipping algorithm.…”

“…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.…”

Bonnsai: a Bayesian tool for comparing stars with stellar evolution models

“…As an initial rotational velocity prior, θ(v ini ), we use observed distributions of stellar rotational velocities such as those found by Hunter et al (2008) Conti & Ebbets (1977), Howarth et al (1997), Abt et al (2002), Martayan et al (2006Martayan et al ( , 2007, Penny & Gies (2009), Huang et al (2010, Dufton et al (2013) or Ramírez-Agudelo et al (2013). We note that the observed distributions of rotational velocities are not necessarily the initial distributions that are actually required as prior function (see de Mink et al 2013).…”

The VLT-Flames Tarantula Survey: an overview of the VFTS results so far

“…Multiplicity properties of the O-type star population van Loon et al (2013) IX. The interstellar medium seen through diffuse interstellar bands and neutral sodium Dufton et al (2013) X. Evidence for a bimodal distribution of rotational velocities for the single early B-type stars Doran et al (2013) XI . Bestenlehner et al, 2011) and a spectrum almost identical to that of R136a3 in the core of the central cluster (Crowther et al, 2010).…”