Context :

Aims : We present N -band spectro-interferometric observations of the red supergiant WOH G64 in the Large Magellanic Cloud ( LMC ) using MIDI at the Very Large Telescope Interferometer ( VLTI ) .
While the very high luminosity ( \sim 5 \times 10 ^ { 5 } L _ { \sun } ) previously estimated for WOH G64 suggests that it is a very massive star with an initial mass of \sim 40 M _ { \sun } , its low effective temperature ( \sim 3200 K ) is in serious disagreement with the current stellar evolution theory .

Methods : WOH G64 was observed with VLTI/MIDI using the UT2-UT3 and UT3-UT4 baseline configurations .

Results : The dust envelope around WOH G64 has been spatially resolved with a baseline of \sim 60 m—the first MIDI observations to resolve an individual stellar source in an extragalactic system .
The observed N -band visibilities show a slight decrease from 8 to \sim 10 \mu m and a gradual increase longward of \sim 10 \mu m , reflecting the 10 \mu m silicate feature in self-absorption .
This translates into a steep increase of the uniform-disk diameter from 8 to 10 \mu m ( from 18 to 26 mas ) and a roughly constant diameter above 10 \mu m. The visibilities measured at four position angles differing by \sim 60° but at approximately the same baseline length ( \sim 60 m ) do not show a noticeable difference , suggesting that the object appears nearly centrosymmetric .
The observed N -band visibilities and spectral energy distribution can be reproduced by an optically and geometrically thick silicate torus model viewed close to pole-on .
The luminosity of the central star is derived to be \sim 2.8 \times 10 ^ { 5 } L _ { \sun } , which is by a factor of 2 lower than the previous estimates based on spherical models .
We also identify the H _ { 2 } O absorption features at 2.7 and 6 \mu m in the spectra obtained with the Infrared Space Observatory and the Spitzer Space Telescope .
The 2.7 \mu m feature originates in the photosphere and/or the extended molecular layers , while the 6 \mu m feature is likely to be of circumstellar origin .

Conclusions : The lower luminosity newly derived from our MIDI observations and two-dimensional modeling brings the location of WOH G64 on the H-R diagram in much better agreement with theoretical evolutionary tracks for a 25 M _ { \sun } star .
However , the effective temperature is still somewhat too cool compared to the theory .
The low effective temperature of WOH G64 places it very close to or even beyond the Hayashi limit , which implies that this object may be experiencing unstable , violent mass loss .