Fitting isochrones to open cluster photometric data III. Estimating metallicities from UBV photometry

TL;DR

This paper introduces an objective isochrone fitting method using the cross-entropy algorithm to estimate open cluster metallicities from UBV photometry, achieving good agreement with spectroscopic data.

Contribution

The study presents a novel, non-subjective approach for simultaneously determining metallicity, distance, reddening, and age of open clusters from photometric data.

Findings

Method yields metallicity estimates with about 0.1 dex precision.

Results agree well with spectroscopic metallicities in literature.

Applicable to 9 open clusters with 15 data sets.

Abstract

The metallicity is a critical parameter that affects the correct determination fundamental characteristics stellar cluster and has important implications in Galactic and Stellar evolution research. Fewer than 10 % of the 2174 currently catalog open clusters have their metallicity determined in the literature. In this work we present a method for estimating the metallicity of open clusters via non-subjective isochrone fitting using the cross-entropy global optimization algorithm applied to UBV photometric data. The free parameters distance, reddening, age, and metallicity simultaneously determined by the fitting method. The fitting procedure uses weights for the observational data based on the estimation of membership likelihood for each star, which considers the observational magnitude limit, the density profile of stars as a function of radius from the center of the cluster, and the density of stars in multi-dimensional magnitude space. We present results of [Fe/H] for nine well-studied open clusters based on 15 distinct UBV data sets. The [Fe/H] values obtained in the ten cases for which spectroscopic determinations were available in the literature agree, indicating that our method provides a good alternative to determining [Fe/H] by using an objective isochrone fitting. Our results show that the typical precision is about 0.1 dex.