Beasor, ER, Davies, B, Smith, N, Gehrz, RD and Figer, DF (2021) The Age of Westerland 1 Revisited. Astrophysical Journal, 912 (1). ISSN 0004-637X

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Abstract

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.

Item Type:	Article
Uncontrolled Keywords:	Science & Technology; Physical Sciences; Astronomy & Astrophysics; INTERSTELLAR EXTINCTION LAW; MASSIVE STELLAR POPULATION; RED SUPERGIANTS; MU-M; INTRINSIC COLORS; MESA ISOCHRONES; STAR-CLUSTERS; EVOLUTION; COOL; VARIABILITY; 0201 Astronomical and Space Sciences; 0202 Atomic, Molecular, Nuclear, Particle and Plasma Physics; 0306 Physical Chemistry (incl. Structural); Astronomy & Astrophysics
Subjects:	Q Science > QB Astronomy Q Science > QC Physics
Divisions:	Astrophysics Research Institute
Publisher:	American Astronomical Society; IOP Publishing
SWORD Depositor:	A Symplectic
Date Deposited:	09 May 2022 10:06
Last Modified:	09 May 2022 10:15
DOI or ID number:	10.3847/1538-4357/abec44
URI:	https://researchonline.ljmu.ac.uk/id/eprint/16775

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