Homogeneous Photometry and Star Counts in the Field of 9 Galactic Star Clusters

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 JHKs 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. © 2009 Elsevier B.V. All rights reserved.AFS acknowledges ESO for supporting a visit to Chile through Director General Discretionary Fundings (DGDF), where this project was completed. EC acknowledges support by the Fondo Nacional de Investigación Cientı´fica y Tecnológica (proyecto No. 1050718 Fondecyt), the Chilean Centro de Astrofı´sica (FONDAP No. 15010003) and the Chilean Centro de Excelencia en Astrofı´sica y Tecnologı´as Afines (PFB 06). The authors are much obliged for the use of the NASA Astrophysics Data System, of theSIMBADdatabase (Centre de Donnés Stellaires – Strasbourg, France) and of the WEBDA open cluster database. This publication also made use of data from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation