[Paper Review] The VLT-FLAMES Tarantula Survey VIII. Multiplicity properties of the O-type star population

H. Sana, A. de Koter+15 more|UvA-DARE (University of Amsterdam)|Sep 20, 2012

Stellar, planetary, and galactic studies41 references230 citations

TL;DR

This study analyzes the multiplicity of 360 O-type stars in the Tarantula Nebula using multi-epoch spectroscopy from the VLT-FLAMES Survey. Applying a Monte Carlo bias correction, it finds an intrinsic binary fraction of 51% with a period distribution favoring short periods and a mass-ratio distribution skewed toward low-mass companions, indicating binary interactions are widespread and critical for massive star evolution.

ABSTRACT

Aims. We analyze the multiplicity properties of the massive O-type star population. With 360 O-type stars, this is the largest homogeneous sample of massive stars analyzed to date. Methods. We use multi-epoch spectroscopy and variability analysis to identify spectroscopic binaries. We also use a Monte-Carlo method to correct for observational biases. Results. We observe a spectroscopic binary fraction of 0.35\pm0.03, which corresponds to the fraction of objects displaying statistically significant radial velocity variations with an amplitude of at least 20km/s. We compute the intrinsic binary fraction to be 0.51\pm0.04. We adopt power-laws to describe the intrinsic period and mass-ratio distributions: f_P ~ (log P)^π (with 0.15 < log P < 3.5) and f_q ~ q^κ with 0.1 < q=M_2/M_1 < 1.0. The power-law indexes that best reproduce the observed quantities are π= -0.45 +/- 0.30 and κ= -1.0\pm0.4. The obtained period distribution thus favours shorter period systems compared to an Oepik law. The mass ratio distribution is slightly skewed towards low mass ratio systems but remains incompatible with a random sampling of a classical mass function. The binary fraction seems mostly uniform across the field of view and independent of the spectral types and luminosity classes. The binary fraction in the outer region of the field of view (r > 7.8', i.e. approx117 pc) and among the O9.7 I/II objects are however significantly lower than expected from statistical fluctuations. Conclusions. Using simple evolutionary considerations, we estimate that over 50% of the current O star population in 30 Dor will exchange mass with its companion within a binary system. This shows that binary interaction is greatly affecting the evolution and fate of massive stars, and must be taken into account to correctly interpret unresolved populations of massive stars.

Motivation & Objective

• To determine the intrinsic multiplicity properties of O-type stars in the Tarantula Nebula, a key starburst region in the Large Magellanic Cloud.
• To correct for observational biases in spectroscopic binary detection due to radial velocity measurement precision and sampling cadence.
• To derive the intrinsic distributions of orbital periods and mass ratios for O-type stars using a multi-dimensional Monte Carlo modeling approach.
• To assess whether the observed binary properties are consistent with formation or evolutionary processes, and to evaluate implications for massive star evolution and compact object formation.
• To test whether the binary fraction and distributions are uniform across the field of view or vary with location, spectral type, luminosity class, or brightness.

Proposed method

• Multi-epoch spectroscopy of 360 O-type stars in the Tarantula Nebula was used to detect radial velocity (RV) variations indicative of spectroscopic binaries.
• A threshold of 20 km s⁻¹ was applied to identify significant RV variations, defining the observed spectroscopic binary fraction.
• A Monte Carlo simulation was employed to model the joint distributions of observed binary fraction, RV amplitude, and timescale of variation, correcting for detection biases.
• The intrinsic period distribution was modeled as a power-law: f(log₁₀P/d) ∝ (log₁₀P/d)⁰·⁴⁵, over the range 0.15 ≤ log₁₀P/d ≤ 3.5.
• The intrinsic mass-ratio distribution was modeled as f(q) ∝ q⁻¹·⁰, for 0.1 ≤ q ≤ 1.0, where q = M₂/M₁.
• The method simultaneously constrained the intrinsic binary fraction, period distribution, and mass-ratio distribution by fitting to the observed data distributions.

Experimental results

Research questions

• RQ1 What is the intrinsic binary fraction of O-type stars in the Tarantula Nebula, corrected for observational biases?
• RQ2 What is the intrinsic distribution of orbital periods among O-type stars, and how does it compare to theoretical expectations such as the Öpik law?
• RQ3 What is the intrinsic distribution of mass ratios in O-type binary systems, and does it favor systems with low-mass companions?
• RQ4 Are the observed multiplicity properties uniform across the field of view, or do they vary with location, spectral type, luminosity class, or brightness?
• RQ5 To what extent do dynamical or evolutionary processes in the Tarantula region affect the observed multiplicity properties, and what does this imply for massive star evolution?

Key findings

• The observed spectroscopic binary fraction is 35% ± 3%, defined as stars showing radial velocity variations with amplitudes ≥20 km s⁻¹.
• After correcting for observational biases using a Monte Carlo method, the intrinsic binary fraction is found to be 51% ± 4%.
• The intrinsic period distribution follows a power-law with index π = -0.45 ± 0.30, favoring shorter-period systems compared to a flat distribution in log₁₀P/d.
• The intrinsic mass-ratio distribution follows f(q) ∝ q⁻¹·⁰, indicating a slight preference for low-mass companions, though constraints are weak (κ = -1.0 ± 0.4).
• The binary fraction is significantly lower in the outer region (r > 7.8′, ≈117 pc) and among the faintest objects (K_s > 15.5 mag), primarily due to sample definition and observational effects.
• The O9.7 I/II stars show a lower binary fraction, suggesting they may be post-interaction stars or part of an older cluster population, though this remains speculative.