BAT99 126: A multiple Wolf-Rayet system in the Large Magellanic Cloud with a massive near-contact binary

TL;DR

BAT99 126 is a complex multiple system in the Large Magellanic Cloud, featuring a short-period eclipsing O+O binary and a Wolf-Rayet star, providing insights into massive star evolution and potential black hole merger progenitors.

Contribution

This study reveals BAT99 126 as a quadruple or higher-order system with detailed characterization of its components and evolutionary status, challenging previous period estimates.

Findings

The system contains at least four components including an eclipsing O+O binary and a Wolf-Rayet star.

The 1.55-day photometric period is due to an O+O binary, not the WR star.

The system is approximately 4.2 million years old and will likely merge before forming black holes.

Abstract

BAT99 126 is a multiple system in the Large Magellanic Cloud containing a Wolf-Rayet (WR) star, which has a reported spectroscopic (orbital) period of 25.5 days and a photometric (orbital) period of 1.55 days, and hence is potentially one of the shortest WR binaries known to date. Such short-period binary systems containing a WR star in low-metallicity environments are prime candidate progenitors of black-hole (BH) mergers. By thoroughly analysing the spectroscopic and photometric data, we aim to establish the true multiplicity of BAT99 126, characterise the orbit(s) of the system, measure the physical properties of its individual components, and determine the overall evolutionary status of the system. Using newly acquired high resolution spectra, we measured radial velocities and performed a spectral analysis of the individual components. We estimated the age of the system and derived an evolutionary scenario for the 1.55-day system. BAT99 126 comprises at least four components. The 1.55-day photometric signal originates in an eclipsing O+O binary of spectral types O4 V and O6.5 V, and masses of 36+/-5 MSun and 15+/-2 MSun, respectively. For the WR star, we derived a spectral type WN2.5-3 and reject the previously reported 25.5-d period. We find evidence in the composite spectra of a fourth component, a B1 V star, which shows radial velocity variation. The configurations of the B-type star, WR star, and possible additional undetected components remain unknown. We estimated the age of the system to be 4.2 Myr. The O+O binary likely went through a phase of conservative mass transfer and is currently a near-contact system. We show that BAT99 126 is a multiple - quadruple or higher-order - system with a total initial mass > 160 MSun. The 1.55-day O+O binary most likely will not evolve towards a BH+BH merger, but instead will merge before the components collapse to BHs.