The dramatic transition of the extreme Red Supergiant WOH G64 to a Yellow Hypergiant

TL;DR

This paper documents the unprecedented transition of the extreme Red Supergiant WOH G64 into a Yellow Hypergiant, revealing real-time stellar evolution and the impact of binarity on massive star final stages.

Contribution

It reports the first observed case of a Red Supergiant transitioning to a Yellow Hypergiant, highlighting the role of binary interactions in late stellar evolution.

Findings

WOH G64 is a rare massive symbiotic binary system.

The star transitioned from RSG to Yellow Hypergiant.

The transition may involve a common-envelope phase or a post-eruption return.

Abstract

Red Supergiants (RSGs) are cool, evolved massive stars in their final evolutionary stage before exploding as a supernova. However, the evolution and fate of the most luminous RSGs remain uncertain. Observational evidence for luminous warm, post-RSG objects and the apparent lack of luminous RSGs as supernova progenitors suggest a blueward evolution. Since the 1980s, WOH G64 has been considered the most extreme RSG in the Large Magellanic Cloud, given its large obscuration, outstanding size, luminosity, and mass-loss rate. Here we report a sudden, yet smooth change in its apparent nature. Time-series photometry and subsequent spectroscopy revealed the most extreme transition ever seen in the optical spectral features of a RSG. We discovered that WOH G64 is a rare, massive symbiotic binary system where the RSG transitioned to a Yellow Hypergiant. The dramatic transition can be explained either by the partial ejection of the pseudo-atmosphere during a common-envelope phase, or the return to a quiescent state after an outstanding eruption exceeding 30 years. WOH G64 offers a unique opportunity to witness stellar evolution in real-time and assess the role of binarity on the final phases of massive stars and their resulting supernovae.