[Paper Review] The VLT-FLAMES Tarantula Survey X: Evidence for a bimodal distribution of rotational velocities for the single early B-type stars

P. L. Dufton, N. Langer+13 more|arXiv (Cornell University)|Dec 11, 2012

Stellar, planetary, and galactic studies59 references65 citations

TL;DR

This study analyzes projected rotational velocities ($v_{\rm{e}}\sin i$) for 334 single early B-type stars in the Tarantula Nebula using high-resolution spectroscopy from the VLT-FLAMES survey. It reveals a bimodal distribution with a slow-rotator component ($v_{\rm{e}} \leq 100$ km s$^{-1}$) comprising 25% of the sample and a fast-rotator component peaking at $\sim$250 km s$^{-1}$, suggesting distinct spin-down mechanisms such as magnetic braking or wind-driven processes.

ABSTRACT

Aims: Projected rotational velocities (\vsini) 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 \kms and show a bi-modal structure. This is also present in the inferred rotational velocity distribution with 25% of the sample having $0\leq$\ve$\leq$100\,\kms and the high velocity component having \ve$\sim 250$\,\kms. 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.

Motivation & Objective

• To determine the intrinsic rotational velocity distribution of single early B-type stars in the 30 Doradus region to understand massive star evolution.
• To investigate whether the observed rotational velocity distribution reflects evolutionary processes, binarity, or magnetic fields.
• To deconvolve the observed $v_{\rm{e}}\sin i$ distribution to infer the true underlying rotational velocity distribution.
• To assess whether the bimodal structure is due to physical mechanisms like magnetic braking or wind-driven spin-down.
• To compare the results with previous surveys and theoretical models of massive star evolution.

Proposed method

• Projected rotational velocities were measured using Fourier transforms of spectral lines, with upper limits derived from profile fitting.
• Non-diffuse and diffuse helium lines were used for narrow- and broad-lined stars, respectively, ensuring consistency across line types.
• The Lucy iterative deconvolution algorithm was applied to transform the observed $v_{\rm{e}}\sin i$ distribution into the intrinsic rotational velocity distribution.
• Stars were selected based on lack of radial velocity variability and absence of supergiant characteristics to isolate single main-sequence stars.
• The analysis focused on early B-type stars (spectral types ~O9.5 to B3) with masses between 6 and 16 $M_\odot$ in the Large Magellanic Cloud.
• Uncertainties in $v_{\rm{e}}\sin i$ estimates were typically ~4%, ensuring high precision in velocity measurements.

Experimental results

Research questions

• RQ1 Does the distribution of projected rotational velocities in single early B-type stars in the Tarantula Nebula exhibit a bimodal structure?
• RQ2 What physical mechanisms—such as magnetic braking, wind-driven spin-down, or binary interactions—could explain the observed bimodal distribution?
• RQ3 Is the bimodal distribution spatially or kinematically correlated with cluster or field populations, or with different episodes of star formation?
• RQ4 How does the intrinsic rotational velocity distribution compare to previous surveys and theoretical models of massive star evolution?
• RQ5 Can the presence of magnetic fields in these stars be linked to the low-velocity component, and what is the origin of such fields?

Key findings

• The observed $v_{\rm{e}}\sin i$ distribution spans up to ~450 km s$^{-1}$ and exhibits a clear bimodal structure.
• The low-velocity component contains approximately 25% of the sample, with $v_{\rm{e}} \leq$ 100 km s$^{-1}$.
• The high-velocity component peaks at $v_{\rm{e}} \sim$ 250 km s$^{-1}$, with most stars in the range 200–350 km s$^{-1}$.
• The bimodal distribution is not correlated with spatial position or radial velocity, ruling out simple field vs. cluster or sequential star formation scenarios.
• The distribution is consistent with theoretical models involving magnetic braking, though alternative mechanisms like bi-stability braking via stellar winds remain plausible.
• Magnetic fields may explain the slow rotators, but the origin of these fields—fossil, dynamo-generated, or binary-induced—remains uncertain.