Towards a photometric metallicity scale for open clusters

Abstract

Context. Open clusters are a useful tool when investigating several topics connected with stellar evolution; for example the age or distance can be more accurately determined than for field stars. However, one important parameter, the metallicity, is only known for a marginal percentage of open clusters. Aims. We aim at a consistent set of parameters for the open clusters investigated in our photometric Δa survey of chemically peculiar stars. Special attention is paid to expanding our knowledge of cluster metallicities and verifying their scale. Methods. Making use of a previously developed method based on normalised evolutionary grids and photometric data, the distance, age, reddening, and metallicity of open clusters were derived. To transform photometric measurements into effective temperatures to use as input for our method, a set of temperature calibrations for the most commonly used colour indices and photometric systems was compiled. Results. We analysed 58 open clusters in total. Our derived metallicity values were in excellent agreement with about 30 spectroscopically studied targets. The mean value of the absolute deviations was found to be 0.03 dex, with no noticeable offset or gradient. The method was also applied using recent evolutionary models based on the currently accepted lower solar abundance value Z ∼ 0.014. No significant differences were found compared to grids using the former adopted solar value Z = 0.02. Furthermore, some divergent photometric datasets were identified and discussed. Conclusions. The method provides an accurate way of obtaining properly scaled metallicity values for open clusters. In light of present and future homogeneous photometric sky surveys, the sample of stellar clusters can be extended to the outskirts of the Milky Way, where spectroscopic studies are almost impossible. This will help for determining galactic metallicity gradients in more detail.

A search for peculiar stars in the open cluster Hogg 16

“…The parameters of Hogg 16 agree very well with the somewhat more populous cluster Collinder 272, which was already analysed with the DG method: log t = 7.3, (m − M ) 0 = 11.75 mag, E(B − V ) = 0.43 mag, and [Fe/H] = 0.03 dex (Netopil & Paunzen 2013;Netopil et al 2016). The distances differ by only 200 pc (∼ 10 %), which is well within typical error ranges even among homogeneous approaches (Netopil et al 2015).…”

“…A comparison with metallicities derived from high-resolution spectroscopic data results in excellent agreement with a scatter of only about 0.05 dex (Heiter et al 2014;Netopil et al 2016). The method was already applied to almost all open clusters of the ∆a survey, confirming a homogeneous scale of the cluster parameters (Netopil & Paunzen 2013).…”

“…Thus, in particular for the B-type cluster stars, the effective temperature as an input to the DG method will be uncertain if adopting a mean reddening. We therefore used the temperature calibration for the reddening-free Q-index by Netopil & Paunzen (2013). Additionally, we selected some fainter (cooler) stars with (b − y), (V − K s ), or (V − I) photometry.…”

“…Additionally, we selected some fainter (cooler) stars with (b − y), (V − K s ), or (V − I) photometry. These data were transformed to effective temperatures by applying the mean cluster reddening given above and the respective reddening ratios and temperature calibrations compiled by Netopil & Paunzen (2013). Finally, mean temperatures were derived for the cluster stars.…”

On the metallicity of open clusters

“…In Fig. 13 we compare the mean metallicities of the final high-resolution sample (Table 11) with the metallicities of Netopil & Paunzen (2013). We have 23 clusters in common with their sample, all of which have metallicities between −0.1 and +0.2 dex.…”