[Paper Review] The Tarantula Massive Binary Monitoring project: II. A first SB2 orbital and spectroscopic analysis for the Wolf-Rayet binary R145

T. Shenar, Noel D. Richardson+20 more|Lirias (KU Leuven)|Oct 24, 2016

Stellar, planetary, and galactic studies104 references22 citations

TL;DR

This study presents the first SB2 orbital solution and spectroscopic disentanglement of the massive Wolf-Rayet binary R145 in the Large Magellanic Cloud, using FLAMES spectroscopy and polarimetry. It reveals a high eccentricity (e ≈ 0.78), mass ratio q ≈ 1.01, and orbital masses of ~55 M☉ each, excluding the primary from being over 100 M☉ and supporting quasi-homogeneous evolution without mass transfer.

ABSTRACT

We present the first SB2 orbital solution and disentanglement of the massive Wolf-Rayet binary R145 (P = 159d) located in the Large Magellanic Cloud. The primary was claimed to have a stellar mass greater than 300Msun, making it a candidate for the most massive star known. While the primary is a known late type, H-rich Wolf-Rayet star (WN6h), the secondary could not be so far unambiguously detected. Using moderate resolution spectra, we are able to derive accurate radial velocities for both components. By performing simultaneous orbital and polarimetric analyses, we derive the complete set of orbital parameters, including the inclination. The spectra are disentangled and spectroscopically analyzed, and an analysis of the wind-wind collision zone is conducted. The disentangled spectra and our models are consistent with a WN6h type for the primary, and suggest that the secondary is an O3.5 If*/WN7 type star. We derive a high eccentricity of e = 0.78 and minimum masses of M1 sin^3 i ~ M2 sin^3 i ~ 13 +- 2 Msun, with q = M2 / M1 = 1.01 +- 0.07. An analysis of emission excess stemming from a wind-wind collision yields a similar inclination to that obtained from polarimetry (i = 39 +- 6deg). Our analysis thus implies M1 = 53^{+40}_{-20} and M2 = 54^{+40}_{-20} Msun, excluding M1 > 300Msun. A detailed comparison with evolution tracks calculated for single and binary stars, as well as the high eccentricity, suggest that the components of the system underwent quasi-homogeneous evolution and avoided mass-transfer. This scenario would suggest current masses of ~ 80 Msun and initial masses of Mi,1 ~ 105 and Mi,2 ~ 90Msun, consistent with the upper limits of our derived orbital masses, and would imply an age of ~2.2 Myr.

Motivation & Objective

• To resolve the long-standing uncertainty about the mass of the primary star in the massive Wolf-Rayet binary R145, previously claimed to exceed 300 M☉.
• To detect and characterize the secondary component, which had not been unambiguously identified in prior studies.
• To derive the complete set of orbital parameters, including orbital inclination, using spectroscopic and polarimetric data.
• To investigate the evolutionary history of the system by comparing observed parameters with theoretical evolution tracks.
• To analyze the wind-wind collision zone and assess its consistency with the derived orbital geometry and stellar parameters.

Proposed method

• Acquisition of high-quality, moderate-resolution FLAMES spectra over multiple epochs to measure radial velocities of both components.
• Application of spectral disentanglement techniques to separate the composite spectra into individual component spectra.
• Simultaneous orbital solution fitting using radial velocity curves to determine orbital elements, including eccentricity and mass function.
• Polarimetric analysis to constrain the orbital inclination via scattering from the wind-wind collision region.
• Spectroscopic analysis of disentangled spectra to determine spectral types, effective temperatures, surface gravities, and mass-loss rates.
• Modeling of wind-wind collision (WWC) features in low-ionization lines to derive the half-opening angle and cross-check the inclination.

Experimental results

Research questions

• RQ1 What is the true mass of the primary star in the R145 binary system, and does it exceed 300 M☉ as previously claimed?
• RQ2 What is the spectral type and physical properties of the secondary component, which had not been clearly detected before?
• RQ3 What is the orbital inclination of the system, and how do polarimetry and wind-wind collision features constrain it?
• RQ4 What evolutionary path best explains the observed mass ratio, eccentricity, and lack of mass transfer in the system?
• RQ5 How consistent are the observed wind-wind collision features with the derived orbital geometry and stellar parameters?

Key findings

• The primary star in R145 is confirmed as a WN6h-type Wolf-Rayet star, with a spectral type consistent with core hydrogen-burning late-type WR.
• The secondary component is identified as an O3.5 If*/WN7-type star, based on disentangled spectral analysis.
• The system has a high orbital eccentricity of e = 0.78 ± 0.03, indicating no significant circularization from mass transfer.
• The mass ratio is q = M₂/M₁ = 1.01 ± 0.07, with minimum masses M₁ sin³i ≈ M₂ sin³i = 13 ± 2 M☉, implying near-equal masses.
• The orbital inclination is constrained to i = 39° ± 6° from polarimetry and i = 40° from WWC modeling, with consistent half-opening angle θ = 76°.
• The derived masses are M₁ = 53⁺⁴⁰₋₂⁰ M☉ and M₂ = 54⁺⁴⁰₋₂⁰ M☉, excluding the possibility of the primary exceeding 100 M☉ and ruling out masses >300 M☉.