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

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 (∼ 5 × 10⁵ L_⊙) previously estimated for WOH G64 suggests that it is a very massive star with an initial mass of ~40 M_⊙, its low effective temperature (~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 ~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 μm and a gradual increase longward of ~10 μm, reflecting the 10 μm silicate feature in self-absorption. This translates into a steep increase of the uniform-disk diameter from 8 to 10 μm (from 18 to 26 mas) and a roughly constant diameter above 10 μm. The visibilities measured at four position angles differing by ~60° 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 × 10⁵ L_⊙, which is by a factor of 2 lower than the previous estimates based on spherical models. We also identify the H2O absorption features at 2.7 and 6 μm in the spectra obtained with the Infrared Space Observatory and the Spitzer Space Telescope. The 2.7 μm feature originates in the photosphere and/or the extended molecular layers, while the 6 μm 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_⊙ star. However, the effective temperature is still somewhat too cool for 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.

Based on observations made with the Very Large Telescope Interferometer of the European Southern Observatory. Program ID: 076.D-0253, 080.D-0222.

This work is based [in part] on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA.