Three open clusters containing Cepheids: NGC 6649, NGC 6664 and Berkeley 55
Journal
Date Issued
2020
Author(s)
Abstract
Classical Cepheids in open clusters play an important role in benchmarking
stellar evolution models, anchoring the cosmic distance scale, and invariably
securing the Hubble constant. NGC 6649, NGC 6664 and Berkeley 55 are three
pertinent clusters that host classical Cepheids and red (super)giants, and an
analysis was consequently initiated to assess newly acquired spectra
($\approx$50), archival photometry, and $Gaia$ DR2 data. Importantly, for the
first time chemical abundances are determined for the evolved members of NGC
6649 and NGC 6664. We find that they are slightly metal-poor relative to the
mean Galactic gradient, and an overabundance of Ba is observed. Those clusters
likely belong to the thin disc, and the latter finding supports D'Orazi et al.
(2009) "$s$-enhanced" scenario. NGC 6664 and Berkeley 55 exhibit radial
velocities consistent with Galactic rotation, while NGC 6649 displays a
peculiar velocity. The resulting age estimates for the clusters ($\approx$70
Ma) imply masses for the (super)giant demographic of $\approx$6 M$_{sun}$.
Lastly, the observed yellow-to-red (super)giant ratio is lower than expected,
and the overall differences relative to models reflect outstanding theoretical
uncertainties.
Volume
644
Start page
A136
Issn Identifier
0004-6361
Ads BibCode
2020A&A...644A.136A
Rights
open.access
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