The cluster Westerlund 1 (Wd1) is host to a large variety of post-main-sequence (MS) massive stars. The simultaneous presence of these stars can only be explained by stellar models if the cluster has a finely tuned age of 4–5 Myr, with several published studies independently claiming ages within this range. At this age, stellar models predict that the cool supergiants (CSGs) should have luminosities of log(L L☉)» 5.5, close to the empirical luminosity limit. Here, we test that prediction using archival data and new photometry from Stratospheric Observatory for Infrared Astronomy to estimate bolometric luminosities for the CSGs. We find that these stars are on average 0.4 dex too faint to be 5 Myr old, regardless of which stellar evolutionary model is used, and instead are indicative of a much older age of 10.4-+1.21.3 Myr. We argue that neither systematic uncertainties in the extinction law nor stellar variability can explain this discrepancy. In reviewing various independent age estimates of Wd1 in the literature, we first show that those based on stellar diversity are unreliable. Second, we reanalyze Wd1’s pre-MS stars employing the Damineli extinction law, finding an age of 7.2-+2.31.1 Myr; older than that of previous studies, but which is vulnerable to systematic errors that could push the age close to 10 Myr. However, there remains significant tension between the CSG age and that inferred from the eclipsing binary W13. We conclude that stellar evolutionary models cannot explain Wd1 under the single-age paradigm. Instead, we propose that the stars in the Wd1 region formed over a period of several megayears.
Bibliographical noteFunding Information:
We thank the referee for a careful reading of our manuscript, and for the suggestions, which improved our paper. Based in part on observations made with the NASA/DLR Stratospheric Observatory for Infrared Astronomy (SOFIA). SOFIA is jointly operated by the Universities Space Research Association, Inc. (USRA), under NASA contract NNA17BF53C, and the Deutsches SOFIA Institut (DSI) under DLR contract 50 OK 0901 to the University of Stuttgart. Financial support for this work was provided by NASA through award No. 05 0064 issued by USRA. E.R.B. is supported by NASA through Hubble Fellowship grant HST-HF2-51428 awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS5-26555. This work makes use of the IDL software and astrolib. R.D.G. was supported by NASA and the United States Air Force.
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