VLT/SINFONI time-resolved spectroscopy of the central, luminous, H-rich WN stars of R136

TL;DR

This study uses VLT/SINFONI and HST/STIS data to investigate the binary status of luminous Wolf-Rayet stars in R136, finding minimal variability and identifying a potential 8.2-day binary system among them.

Contribution

First time time-resolved NIR spectroscopy combined with archival imaging to assess binarity of R136's WR stars, challenging previous claims of a 4.377-day binary.

Findings

No significant photometric variability detected.

One star shows marginal radial velocity variability.

Potential 8.2-day binary system identified.

Abstract

Using the Very Large Telescope's Spectrograph for INtegral Field Observation in the Near-Infrared (VLT/SINFONI), we have obtained repeated AO-assisted, NIR spectroscopy of the six central luminous, Wolf-Rayet (WR) stars in the core of the very young (~1 Myr), massive and dense cluster R136, in the Large Magellanic Cloud (LMC). We also de-archived available images that were obtained with the Hubble Space Telescope's Space Telescope Imaging Spectrograph (HST/STIS), and extracted high-quality, differential photometry of our target stars to check for any variability related to binary motion. Previous studies, relying on spatially unresolved, integrated, optical spectroscopy, had reported that one of these stars was likely to be a 4.377-day binary. Our study set out to identify the culprit and any other short-period system among our targets. However, none displays significant photometric variability, and only one star, BAT99-112 (R136c), located on the outer fringe of R136, displays a marginal variability in its radial velocities; we tentatively report an 8.2-day period. The binary status of BAT99-112 is supported by the fact that it is one of the brightest X-ray sources among all known WR stars in the LMC, consistent with it being a colliding-wind system. Follow-up observations have been proposed to confirm the orbital period of this potentially very massive system.