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

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. 1 The latest version (3.5) can be accessed on line at https:// wilton.unifei.edu.br/ocdb/.has contributed in this topic through membership probability estimates from both photometric (Monteiro et al. 2017) and proper motion data (Dias et al. 2014(Dias et al. , 2018. In this study we show how the solution of this question evolved in the era of the Gaia mission (Gaia Collaboration et al. 2018a).The open clusters investigated in the present study were selected from DAML02 with the aim to provide a set of homogeneous UBVRI data and parameters determined from them in a non subjective isochrone-fitting approach. For all the clusters studied here, except for NGC 6087, these are the first results based on UBVRI data. The cluster NGC 6087 was observed in this run because the Cepheid S Nor, member of the cluster, allows us to obtain an independent check of the distance estimate by our isochrone fit method.Despite the fact that several of the clusters investigated in the present work had previous determinations of distances and ages, important corrections were needed given the discrepancies between results obtained by different works using different methods, as discussed by Netopil et al. (2015).

The spiral pattern rotation speed of the Galaxy and the corotation radius with Gaia DR2

“…Because of the large volume of data in Gaia DR2 and the large number of clusters to be analyzed, the resolution of the equations above can take a considerable amount of computational time due to the matrix operations involved. So, we opted to estimate the membership in two steps, as performed in Dias et al (2018b) and Monteiro and Dias (2019). First we use the proper motion following the procedure described in Dias et al (2018a) to select the stars with membership greater than 0.50 based on that model.…”

“…The main objective was to determine the parameters: distance, age, color-excess and metallicity. The method to obtain distances and ages by isochrone fitting is described in previous papers of our group, applied to UBVRI data (Caetano et al 2015) and (Monteiro et al 2017) and more recently, applied to Gaia DR2 data (Dias et al 2018b) and Monteiro and Dias (2019). We used exactly the same procedures in this work.…”

“…For a number of open clusters of the final sample, we determined values of distance and age which are in disagreement with published ones. A detailed discussion of each case is unnecessary, since Dias et al (2018b) and the results obtained for the control sample from Moitinho (2001) show that the procedures and results obtained with Gaia DR2 data are reliable. In addition, the CMDs with our fitted isochrones were clearly better than those from the literature.…”

The nature of the NGC 2546: Not one but two open clusters

“…This work shows how productive is to combine data obtained through different observational techniques to perform a comprehensive analysis of a system of interest. In particular, this paper, alike a number of recent studies of open clusters (e.g., Dias et al 2018;Fritzewski et al 2019), demonstrates the great usefulness of the precise Gaia data to determine age and distance of stellar clusters with scarce available information, as well as to obtain a better picture of their spatial dimension and structure.…”

An analysis of four stellar rings

“…The proper motions of our targets based on the Gaia DR2 are presented in Figure . Let us keep in mind that the typical spread of the proper motions for members of open clusters and stellar moving groups is about ±3 mas year –1 (Dias et al ; Röser et al ). This heuristically observed value is due to the separation of the primordial molecular cloud and the conservation of angular momentum.…”