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

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.

Figures (11)

• Figure 1: Light curves of WOH G64. Each dataset is labeled with the corresponding survey and filter. NEOWISE-R W1 data is shown with a +7.4 mag offset. The gray patch between 2013.5 and 2014.5 constrains the epoch of the transition. The vertical lines indicate the epochs of spectroscopy and the corresponding optical spectroscopic classifications reported for WOH G64 (Table \ref{['tab:spectral_class']}).
• Figure 1: Comparison of observed color–magnitude variability with synthetic photometry. Left: Photometric observations are shown with dots (OGLE, between January 2010 and October 2017) and squares (MACHO, between June 1993 and December 1999). The dotted line represents the MACHO synthetic colors for a model with $\textit{T$_{\rm eff}$} = 3300$ K and variable $A_V$, while the dot-dashed line corresponds to OGLE synthetic colors for a model with variable $\textit{T$_{\rm eff}$}$ and a fixed $A_V = 4$ mag. Black arrows indicate the direction of increasing $A_V$ and $T_{\rm eff}$ in the diagram. Shaded areas represent the full range of observed color variation. Top right: Values of $A_V$ required to reproduce the observed MACHO colors using synthetic photometry, as a function of $T_{\rm eff}$. Bottom right: Values of $T_{\rm eff}$ required to reproduce the observed OGLE colors using synthetic photometry, as a function of $A_V$.
• Figure 2: Evolution of the optical colors of WOH G64. MACHO $V_{\rm KC}-R_{\rm KC}$ is shown in red squares, OGLE $V-I$ in magenta circles, and Gaia$BP-RP$ in black crosses. $R_{\rm KC}$ and $I$ are represented in gray squares and circles, respectively, with an offset for comparison. Vertical dashed lines indicate the optical spectral classifications. The estimated time of the transition between 2013.5 and 2014.5 is highlighted with the shaded area.
• Figure 2: Comparison of the optical spectra of WOH G64. Top: Spectral region between 6200-7200 Å. Bottom: Spectral region of the Ca ii triplet region. The X-Shooter (2016) and MagE (2021) spectra are normalized, and the UVES spectrum is shown with an offset. The main spectral features are indicated for each epoch.
• Figure 3: Optical spectra of WOH G64 illustrating its dramatic transition from a late-M star to a B[e] star. The main spectral features are indicated for each epoch. The spectra were scaled to 1 between 8700-8800 Å. The UVES spectrum is shown with an offset and downgraded to a spectral resolution of 4000.