Spatially resolved dusty torus toward the red supergiant WOH G64 in the Large Magellanic Cloud

Abstract

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). The location of WOH G64 on the H-R diagram based on the previously estimated luminosities is in serious disagreement with the current stellar evolution theory. The dust envelope around WOH G64 has been spatially resolved with a baseline of ~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 10 micron and a gradual increase longward of 10 micron, reflecting the 10 micron silicate feature in self-absorption. The visibilities measured at four position angles differing by ~60 degrees but at approximately the same baseline length (~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 2.8 x 10^5 Lsun, which is by a factor of 2 lower than the previous estimates based on spherical models. 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 Msun star. We also identify the H2O absorption features at 2.7 and 6 micron in the spectra obtained with the Infrared Space Observatory and the Spitzer Space Telescope. The 2.7 micron feature originates in the photosphere and/or the extended molecular layers, while the 6 micron feature is likely to be of circumstellar origin.