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Three open clusters containing Cepheids: NGC 6649, NGC 6664, and Berkeley 55

J. Alonso-Santiago, I. Negueruela, A. Marco, H. M. Tabernero, N. Castro

Abstract
Classical Cepheids in open clusters play an important role in benchmarking stellar evolution models, in addition to anchoring the cosmic distance scale and invariably securing the Hubble constant. Three pertinent clusters hosting classical Cepheids and red (super)giants are: NGC 6649, NGC 6664, and Berkeley 55. These clusters form the basis of analysis to assess newly acquired spectra (≈50), archival photometry, and Gaia DR2 data. Importantly, for the first time chemical abundances were 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. Also, an overabundance of Ba is observed. These two clusters likely belong to the thin disc and the latter finding supports the “s-enhanced” scenario of D’Orazi et al. (2009). 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 (≈70 Ma) imply masses of ≈6 M for the (super)giant demographic. Lastly, the observed yellow-to-red (super)giant ratio is lower than expected and the overall differences that are relative to the models reflect the outstanding theoretical uncertainties.

Keywords
open clusters and associations: individual: NGC 6649; open clusters and associations: individual: NGC 6664; open clusters and associations: individual: Berkeley 55; Hertzsprung-Russell and C-M diagrams; stars: abundances; stars: variables: Cepheids; Astro

Astronomy and Astrophysics
Volume 644, Article Number A136, Number of pages 22
2020 December

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Instituto de Astrofísica e Ciências do Espaço Universidade do Porto Faculdade de Ciências da Universidade de Lisboa
Fundação para a Ciência e a Tecnologia COMPETE 2020 PORTUGAL 2020 União Europeia