Homogeneous photometry and star counts in the field of 9 Galactic star clusters

TL;DR

This study provides homogeneous photometric data and analysis for nine Galactic star fields, revealing their nature, size, and fundamental parameters, and identifying several as intermediate or old age clusters, including some previously overlooked ones.

Contribution

The paper offers new homogeneous $V,I$ CCD photometry and analysis of nine Galactic fields, establishing their cluster status and properties, and identifying previously overlooked old clusters.

Findings

Most clusters are projected towards the Carina-Sagittarius spiral arm.

Trumpler 20 is older than previously thought, with a double red clump.

Hogg 19 is a newly identified old open cluster in a complex Galactic region.

Abstract

We present homogeneous $V,I$ CCD photometry of nine stellar fields in the two inner quadrants of the Galactic plane. The lines-of-view to most of these fields aim in the direction of the very inner Galaxy, where the Galactic field is very dense, and extinction is high and patchy. Our nine fields are, according to several catalogs, centred on Galactic star clusters, namely Trumpler 13, Trumpler 20, Lynga 4, Hogg 19, Lynga 12, Trumpler 25, Trumpler 26, Ruprecht 128, and Trumpler 34. Apart from their coordinates, and in some cases additional basic data (mainly from the 2MASS archive), their properties are poorly known. By means of star count techniques and field star decontaminated Color-Magnitude diagrams, the nature and size of these visual over-densities has been established; and, when possible, new cluster fundamental parameters have been derived. To strengthen our findings, we complement our data-set with JHK$_{s}$ photometry from the 2MASS archive, that we analyze using a suitably defined Q-parameter. Most clusters are projected towards the Carina-Sagittarium spiral arm. Because of that, we detect in the Color Magnitude Diagrams of most of the other fields several distinctive sequences produced by young population within the arm. All the clusters are of intermediate or old age. The most interesting cases detected by our study are, perhaps, that of Trumpler 20, which seems to be much older than previously believed, as indicated by its prominent -and double- red clump; and that of Hogg 19, a previously overlooked old open cluster, whose existence in such regions of the Milky Way is puzzling.