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

TL;DR

This study analyzes three open clusters with Cepheids, providing new chemical abundance data, assessing their galactic membership, and comparing observed stellar populations with theoretical models to improve understanding of stellar evolution and distance calibration.

Contribution

First-time chemical abundance measurements for NGC 6649 and NGC 6664, and a comprehensive analysis of their stellar populations and galactic context.

Findings

NGC 6649 and NGC 6664 are slightly metal-poor and Ba overabundant.

Clusters likely belong to the thin disc, with NGC 6649 showing peculiar velocity.

Observed stellar ratios and properties reveal uncertainties in current stellar evolution models.

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.