Resolving the Core of R136 in the Optical

Abstract

The sharpest optical images of the R136 cluster in the Large Magellanic Cloud are presented, allowing us for the first time to resolve members of the central core, including R136a1, the most-massive star known. These data were taken using the Gemini speckle imager Zorro in medium-band filters with effective wavelengths similar to BVRI achieving angular resolutions between 30-40 mas. All stars previously known in the literature, having V &lt; 16 mag within the central 2″ × 2″, were recovered. Visual companions (≥40 mas; 2000 au) were detected for the WN5h stars R136 a1 and a3. Photometry of the visual companion of a1 suggests it is of mid-O spectral type. Based on new photometric luminosities using the resolved Zorro imaging, the masses of the individual WN5h stars are estimated to be between 150 and 200 M <SUB>⊙</SUB>, lowering significantly the present-day masses of some of the most-massive stars known. These mass estimates are critical anchor points for establishing the stellar upper-mass function.