On the metallicity of open clusters

Abstract

Context. Open clusters are an important tool for studying the chemical evolution of the Galactic disk. Metallicity estimates are available for about ten percent of the currently known open clusters. These metallicities are based on widely differing methods, however, which introduces unknown systematic effects. Aims. In a series of three papers, we investigate the current status of published metallicities for open clusters that were derived from a variety of photometric and spectroscopic methods. The current article focuses on spectroscopic methods. The aim is to compile a comprehensive set of clusters with the most reliable metallicities from high-resolution spectroscopic studies. This set of metallicities will be the basis for a calibration of metallicities from different methods. At medium and high resolution, we found that differences in the analysis methods have a stronger effect on the metallicity than that of quality differences in the observations. We retained only highly probable cluster members and introduced a restriction on atmospheric parameters. Results. We combined 641 individual metallicity values for 458 stars in 78 open clusters from 86 publications to form our final set of high-quality cluster metallicities. The photometric metallicities discussed in the first paper of this series are systematically lower than the spectroscopic ones by about 0.1 dex, and the differences show a scatter of about 0.2 dex. In a preliminary comparison of our spectroscopic sample with models of Galactic chemical evolution, none of the models predicts the observed radial metallicity gradient. Conclusions. Photometric metallicities show a large intrinsic dispersion, while the more accurate spectroscopic sample presented in this paper comprises fewer than half the number of clusters. Only a sophisticated combination of all available photometric and spectroscopic data will allow us to trace the metallicity distribution in the Galactic disk on a local and global scale.

Isochrones of M67 with an Expanded Set of Parameters

“…as in Sarajendini et al, 2009) at an age of around 4 Gyr (Richer et al, 1998). The cluster has a metallicity around solar, with [Fe/H] = 0.00 ± 0.06 (Heiter et al, 2014) and a reddening of E(B − V) = 0.041 ± 0.004 (Taylor 2007). A color magnitude diagram (CMD) of M67 can be seen in Figure 1 (data from Geller et al, 2015).…”

Testing the chemical tagging technique with open clusters

“…To reduce the dataset to mainly FGK stars in the main sequence and red giant branch, we discarded spectra for which we found an effective temperature higher than 6500 K or lower than 4500 K, and a surface gravity higher than 4.60 dex or lower than 2.00 dex (same limits as in Heiter et al 2014). After this third selection criterion, 389 spectra remain, which correspond to 339 stars.…”

The evolution of the Milky Way: new insights from open clusters

“…However, not all the investigators have arrived at similar measures of the radial metallicity gradient which may be partly related to the different methods adopted (see for example, Magrini et al 2009;Heiter et al 2014). Our current understanding suggests a gradient of about −0.06 dex kpc −1 (Friel et al 2002;Pancino et al 2010) to −0.20 dex kpc −1 (Frinchaboy et al 2013) in the radial range 5 to 10 kpc with a nearly flat trend (−0.02 dex kpc −1 ) beyond 13 kpc (Carraro et al 2007;Sestito et al 2008;Pancino et al 2010;Yong et al 2012;Frinchaboy et al 2013;Cantat-Gaudin et al 2016).…”