Abstract

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 (similar to 5 x 10(5) L(circle dot)) previously estimated for WOH G64 suggests that it is a very massive star with an initial mass of similar to 40 M(circle dot), its low e. ective temperature (similar to 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 similar to 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 similar to 10 mu m and a gradual increase longward of similar to 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 similar to 60 degrees but at approximately the same baseline length (similar to 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 similar to 2.8 x 10(5) L(circle dot), 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(circle dot) star. However, the effective temperature is still somewhat too cool for the theory. The low e. ective 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.