Astrometric and photometric study of Dias 4, Dias 6 and other five open clusters using ground based and Gaia DR2 data

TL;DR

This study combines ground-based UBVRI photometry and Gaia DR2 data to analyze seven southern open clusters, determining their fundamental parameters and discovering two new clusters.

Contribution

First-time UBVRI photometry for several clusters and a comprehensive analysis using Gaia DR2 data with a novel multidimensional optimization approach.

Findings

Gaia DR2 data yields more precise cluster parameters.

Confirmed distance to NGC 6087 using Cepheid S Nor.

Discovered two new clusters near Dias 4.

Abstract

We present a study of 7 southern open clusters based on UBVRI CCD photomety (Johnsons-Cousins system) and Gaia DR2 data. Dias 4, Dias 6 and four other clusters had UBVRI photometric observations determined for the first time. From the observational UBVRI data we obtained photometric membership probability estimates and, using the proper motions from the UCAC5 catalog, we also determined the kinematic membership. From Gaia DR2 astrometric data we determine the stellar membership using proper motions and parallaxes, taking into account the full covariance matrix. For both independent sets of data and membership we apply our non subjective multidimensional global optimization tool to fit theoretical isochrones to determine the distance, age, reddening, metallicity and binary fraction of the clusters. The results of the mean proper motions, distances and ages are in agreement, but the ones obtained from Gaia DR2 data are more precise in both membership selection and estimated parameters. In the case of NGC 6087, the Cepheid S Nor, member of the open cluster, was used to obtain an independent distance estimate, confirming the one determined by our fitting method. We also report a serendipitous discovery of two new clusters in the extended field near what was originally Dias 4.