A phoenix rises from the ashes: WOH G64 is still a red supergiant, for now

TL;DR

Despite recent dramatic changes, WOH G64 remains a red supergiant, with spectral evidence suggesting it may never have transitioned into a yellow hypergiant, providing new insights into its evolutionary state.

Contribution

This study presents new spectral observations that challenge previous claims of WOH G64's transition to a yellow hypergiant, confirming its continued status as a red supergiant.

Findings

Spectra show persistent TiO absorption bands indicating red supergiant status.

Detection of VO suggests a highly extended atmosphere.

Atomic lines are deepening, constraining evolutionary scenarios.

Abstract

For a long time, WOH G64 was known as the most extreme red supergiant outside our Galaxy. However, in a matter of years it has faded, its pulsations have become suppressed and the spectrum has become dominated by emission lines from ionised gas, a far cry from the Mira-like pulsation and late M-type spectrum it used to display. Around the same time, a hot dust cloud was discovered using the VLT interferometer. WOH G64 has been claimed to have turned into a yellow hypergiant, which could signal a pre-supernova post-red supergiant evolution. Here we present spectra of WOH G64 obtained with the Southern African Large Telescope (SALT) between November 2024 and December 2025. Molecular absorption bands from TiO are seen at all times. This implies that WOH G64 is currently a red supergiant, and may never have ceased to be. However, the shallow, resolved bands and possible detection of VO hint at a highly extended atmosphere. The continuum appears to be varying, while the line emission shows a different behaviour, suggesting two separate components in the system. Meanwhile, atomic absorption lines are deepening. This places important constraints on scenarios for the dramatic events that are unfolding.