[Paper Review] The Tarantula Massive Binary Monitoring: I. Observational campaign and OB-type spectroscopic binaries

L. A. Almeida, H. Sana+25 more|White Rose Research Online (University of Leeds, The University of Sheffield, University of York)|Oct 11, 2016

Stellar, planetary, and galactic studiesPhysics and Astronomy45 references55 citations

TL;DR

This study presents the first comprehensive radial velocity monitoring of 102 massive binaries in the 30 Doradus region using 32 FLAMES/GIRAFFE observations, deriving orbital solutions for 82 systems (51 SB1s and 31 SB2s). It finds that the orbital period distribution in the Large Magellanic Cloud is remarkably similar to that in the Milky Way, suggesting minimal environmental influence on massive binary properties despite metallicity differences.

ABSTRACT

Massive binaries (MBs) play a crucial role in the Universe. Knowing the distributions of their orbital parameters (OPs) is important for a wide range of topics, from stellar feedback to binary evolution channels, from the distribution of supernova types to gravitational wave progenitors, yet, no direct measurements exist outside the Milky Way. The Tarantula Massive Binary Monitoring was designed to help fill this gap by obtaining multi-epoch radial velocity monitoring of 102 MBs in the 30 Dor. In this paper, we analyse 32 VLT/FLAMES observations of 93 O- and 7 B-type binaries. We performed a Fourier analysis and obtained orbital solutions for 82 systems: 51 single- and 31 double-lined spectroscopic binaries. Overall, the OPs and binary fraction are remarkably similar across the 30 Dor region and compared to existing Galactic samples (GSs). This indicates that within these domains environmental effects are of second order in shaping the properties of MBs. A small difference is found in the distribution of orbital periods (OrbPs), which is slightly flatter (in log space) in 30 Dor than in the Galaxy, although this may be compatible within error estimates and differences in the fitting methodology. Also, OrbPs in 30 Dor can be as short as 1.1 d; somewhat shorter than seen in GSs. Equal mass binaries q>0.95 in 30 Dor are all found outside NGC 2070 the very young and massive cluster at 30 Dor's core. One outstanding exception however is the fact that earliest spectral types (O2-O7) tend to have shorter OrbPs than latter (O9.2-O9.7). Our results point to a relative universality of the incidence rate of MBs and their OPs in the metallicity range from solar ($Z_{\odot}$) to about $0.5Z_{\odot}$. This provides the first direct constraints on MB properties in massive star-forming galaxies at the Universes peak of star formation at redshifts z~1 to 2, which are estimated to have $Z~0.5Z_{\odot}$.

Motivation & Objective

• To measure the orbital parameters of massive binary systems in the 30 Doradus region, a key site for studying massive star formation outside the Milky Way.
• To assess the impact of low metallicity (LMC) on the distribution of orbital periods and eccentricities in massive binaries compared to Galactic samples.
• To investigate spatial variations in binary properties across different subregions of 30 Doradus, such as NGC 2070 and NGC 2060, to probe environmental and evolutionary effects.
• To identify and characterize equal-mass binaries and their spatial distribution, particularly in relation to the central R136a cluster.
• To lay the foundation for future spectral disentangling and population synthesis modeling to infer initial binary distributions and evolutionary pathways.

Proposed method

• Conducted multi-epoch radial velocity monitoring using the FLAMES/GIRAFFE spectrograph on the VLT over 2013–2014.
• Performed Fourier analysis on radial velocity curves to detect periodic signals and derive orbital solutions for spectroscopic binaries.
• Classified systems as single-lined (SB1) or double-lined (SB2) spectroscopic binaries based on spectral line behavior.
• Corrected for detection biases using statistical modeling to compare the observed period distribution with Galactic samples.
• Analyzed spatial variations in orbital parameters across distinct regions: NGC 2070 (central cluster), NGC 2060 (older cluster), and field regions.
• Used spectral disentangling techniques (planned for future work) to recover individual stellar spectra and improve mass ratio measurements.

Experimental results

Research questions

• RQ1 How do the orbital period distributions of massive binaries in the 30 Doradus region compare to those in the Milky Way, particularly given the metallicity difference?
• RQ2 What is the role of environmental factors such as cluster density and age in shaping the orbital properties of massive binaries in 30 Doradus?
• RQ3 Are equal-mass binaries (q > 0.95) preferentially located outside the central R136a cluster, and what does this imply about binary evolution?
• RQ4 Do earlier spectral type O stars (O2–O7) have systematically shorter orbital periods than later types (O9.2–O9.7), and what might cause this trend?
• RQ5 To what extent do multiplicity and spectral classification biases affect the luminosity class distribution of massive stars?

Key findings

• The orbital period distribution in 30 Doradus is remarkably similar to that in the Milky Way, with a power-law index of -0.1 in log space between 1 and 1000 days, indicating minimal metallicity-driven differences.
• The distribution of orbital periods in 30 Doradus is slightly flatter than in Galactic samples, though this difference is small and potentially within error or fitting method uncertainties.
• Orbital periods as short as 1.1 days are observed in 30 Doradus, slightly shorter than the shortest periods seen in Galactic samples.
• All equal-mass binaries (q > 0.95) are found outside NGC 2070, the central association around R136a, suggesting dynamical or evolutionary suppression of such systems in dense environments.
• Earlier spectral type O stars (O2–O7) tend to have shorter orbital periods than later O-type stars (O9.2–O9.7), a trend possibly linked to initial mass function or binary evolution effects.
• The eccentricity distribution in NGC 2070 differs slightly from the field, and NGC 2060 shows a higher fraction of SB1 systems, both of which are qualitatively consistent with binary evolution expectations.