Don’t Believe the Hype(r): The Yellow Supergiants of Westerlund 1

Abstract Yellow hypergiants (YHGs) are often presumed to represent a transitional post-red supergiant (RSG) phase for stars ∼30–40 M ⊙ . Here we present visual-wavelength echelle spectra of six YHG candidates in the Galactic cluster Westerlund 1, and compare them known YHGs, IRC +10420 Hen 3-1979. We find that do not exhibit any metal emission lines, nor they show strong H α emission, as such meet criteria necessary be classified YHGs. In conjunction with their moderate luminosities log ( L stretchy="true">/ ⊙ stretchy="false">) = 4.7–5.4 estimated from optical/infrared photometry, suggest instead normal yellow supergiants (YSGs) more modest initial masses around 15–20 This adds additional support hypothesis Wd1 is multiage an older age than previously assumed, ∼5 Myr old caught at very specific point when single-star evolution might yield Wolf–Rayet stars, luminous blue variables, RSGs, YHGs same cluster. Nevertheless, population YSGs unusual, outnumbering but both spanning large luminosity range. Here, discuss evolutionary scenarios have led high fraction YSGs. The number significant spread cannot explained by simple synthesis models single or binary stars. Even multiple ages spread, YSG/RSG ratio remains problematic. objects may experience prolonged YSG due triple systems.