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

O. H. Ramírez-Agudelo, S. Simón‐Díaz+16 more|UvA-DARE (University of Amsterdam)|Sep 11, 2013

Stellar, planetary, and galactic studiesPhysics and Astronomy69 references113 citations

TL;DR

This study analyzes the projected rotational velocities (vₑsin i) of 216 presumably single O-type stars in the 30 Doradus region using high-resolution spectroscopy from the VLT-FLAMES Tarantula Survey. It reveals a bimodal distribution with a low-velocity peak at ~80 km s⁻¹ and a high-velocity tail extending to ~600 km s⁻¹, indicating that 20% of the sample are fast rotators, suggesting binary interaction may dominate the high-speed population and challenging the single-star origin of long-duration gamma-ray bursts.

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, $ m{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 $ m{P(\veq)}$ with around 80% of the sample having 0 $

Motivation & Objective

• To determine the distribution of projected rotational velocities (vₑsin i) for a large, homogeneous sample of presumably single O-type stars in the 30 Doradus region.
• To investigate variations in rotational velocity across sub-populations defined by spectral type, luminosity class, and spatial location within the field of view.
• To infer the intrinsic equatorial rotational velocity distribution P(vₑ) using iterative deconvolution to assess the role of stellar winds and angular momentum loss in spin-down mechanisms.
• To evaluate the implications of the observed rotation distribution for the evolutionary origin of long-duration gamma-ray burst (LGRB) progenitors.
• To assess whether the high-velocity tail is consistent with single-star evolution or primarily due to post-binary interaction products.

Proposed method

• Measured vₑsin i using a Fourier transform method and a profile fitting method on isolated spectral lines from high-resolution optical spectra.
• Applied an iterative deconvolution procedure to reconstruct the intrinsic probability density function P(vₑ) of equatorial rotational velocities from the observed vₑsin i distribution.
• Used a sample of 216 O-type stars selected as single based on radial velocity stability and absence of spectroscopic binary signatures.
• Analyzed the rotational velocity distribution as a function of spectral subtype, luminosity class, and spatial position within the 30 Doradus field.
• Compared the inferred P(vₑ) distribution with theoretical expectations from single-star evolution and binary interaction models.
• Evaluated the implications of the high-velocity tail for LGRB progenitor scenarios, particularly the role of metallicity and binary evolution.

Experimental results

Research questions

• RQ1 What is the intrinsic distribution of equatorial rotational velocities (P(vₑ)) for single O-type stars in 30 Doradus, and does it exhibit bimodal structure?
• RQ2 To what extent do stellar winds and mass loss in single O-type stars account for the observed spin-down from initial critical rotation?
• RQ3 Is the high-velocity tail in vₑsin i (up to ~600 km s⁻¹) consistent with single-star evolution or primarily due to post-binary interaction?
• RQ4 What fraction of the O-star population in 30 Doradus has vₑ > 300 km s⁻¹, and how does this fraction vary across spectral subtypes and spatial locations?
• RQ5 Does the presence of fast rotators challenge the requirement of low-metallicity environments for long-duration gamma-ray burst progenitors?

Key findings

• The vₑsin i distribution exhibits a bimodal structure with a prominent peak at ~80 km s⁻¹ and a high-velocity tail extending up to ~600 km s⁻¹.
• Approximately 80% of the sample have equatorial rotational velocities (vₑ) ≤ 300 km s⁻¹, while the remaining 20% are fast rotators with vₑ > 300 km s⁻¹.
• The inferred P(vₑ) distribution confirms the bimodal nature, with the low-velocity peak consistent with previous LMC surveys and the high-velocity tail indicating a significant population of rapidly rotating stars.
• The observed rotational velocity distribution implies that stellar winds alone are insufficient to spin down O-type stars from near-critical to observed rotation rates within the first few million years of evolution.
• The high-velocity tail is likely dominated by post-binary interaction products, such as merged binaries or products of mass transfer, based on the lack of radial velocity variability in the sample.
• If the high-velocity tail is exclusively composed of binary interaction products, the single-star channel for long-duration gamma-ray burst progenitors is significantly diminished or ruled out, unless binarity itself is metallicity-dependent.