Imaging the innermost circumstellar environment of the red supergiant WOH G64 in the Large Magellanic Cloud

TL;DR

This study used near-infrared interferometry to image the innermost circumstellar environment of the red supergiant WOH G64 in the Large Magellanic Cloud, revealing new dust formation and structural features.

Contribution

First interferometric imaging of an extragalactic RSG's innermost environment, showing changes in dust and spectral features over time.

Findings

Revealed elongated compact emission around WOH G64.

Detected significant spectral change indicating new dust formation.

Observed discrepancy in stellar flux contribution compared to previous dust models.

Abstract

Significant mass loss in the red supergiant (RSG) phase has great influence on the evolution of massive stars and their final fate as supernovae. We present near-infrared interferometric imaging of the circumstellar environment of the dust-enshrouded RSG WOH G64 in the Large Magellanic Cloud. WOH G64 was observed with the GRAVITY instrument at ESO's Very Large Telescope Interferometer (VLTI) at 2.0--2.45 micron. We succeeded in imaging the innermost circumstellar environment of WOH G64 -- the first interferometric imaging of an RSG outside the Milky Way. The reconstructed image reveals elongated compact emission with a semimajor and semiminor axis of ~2 and ~1.5 mas (~13 and 9 stellar radii), respectively. The GRAVITY data show that the stellar flux contribution at 2.2 micron at the time of our observations in 2020 is much lower than predicted by the optically and geometrically thick dust torus model based on the VLTI/MIDI data taken in 2005 and 2007. We found a significant change in the near-infrared spectrum of WOH G64: while the (spectro)photometric data taken at 1--2.5 micron before 2003 show the spectrum of the central RSG with H2O absorption, the spectra and JHK' photometric data taken after 2016 are characterized by a monotonically rising continuum with very weak signatures of H2O. This spectral change likely took place between December 2009 and 2016. On the other hand, the mid-infrared spectrum obtained in 2022 with VLT/VISIR agrees well with the spectra obtained before 2007. The compact emission imaged with GRAVITY and the near-infrared spectral change suggest the formation of hot new dust close to the star, which gives rise to the monotonically rising near-infrared continuum and the high obscuration of the central star. The elongation of the emission may be due to the presence of a bipolar outflow or effects of an unseen companion.